Mobility of cytochrome P450 in the endoplasmic reticulum membrane

Exploring the biocatalysis of psilocybin and other tryptamines: Enzymatic pathways, synthetic strategies, and industrial implications

Tryptamines play diverse roles as neurotransmitters and psychoactive compounds found in various organisms. Psilocybin, a notable tryptamine, has garnered attention for its therapeutic potential in treating mental health disorders like depression and anxiety. Despite its promising applications, current extraction methods for psilocybin are labor-intensive and economically limiting. We suggest biocatalysis as a sustainable alternative, leveraging enzymes to synthesize psilocybin and other tryptamines efficiently. By elucidating psilocybin biosynthesis pathways, researchers aim to advance synthetic methodologies and industrial applications. This review underscores the transformative potential of biocatalysis in enhancing our understanding of tryptamine biosynthesis and facilitating the production of high-purity psilocybin and other tryptamines for therapeutic and research use.

Discovery of a monomeric green fluorescent protein sensor for chloride by structure-guided bioinformatics

Chloride is an essential anion for all forms of life. Beyond electrolyte balance, an increasing body of evidence points to new roles for chloride in normal physiology and disease. Over the last two decades, this understanding has been advanced by chloride-sensitive fluorescent proteins for imaging applications in living cells. To our surprise, these sensors have primarily been engineered from the green fluorescent protein (GFP) found in the jellyfish Aequorea victoria. However, the GFP family has a rich sequence space that could already encode for new sensors with desired properties, thereby minimizing protein engineering efforts and accelerating biological applications. To efficiently sample this space, we present and validate a stepwise bioinformatics strategy focused first on the chloride binding pocket and second on a monomeric oligomerization state. Using this, we identified GFPxm163 from GFPxm found in the jellyfish Aequorea macrodactyla. In vitro characterization shows that the binding of chloride as well as bromide, iodide, and nitrate rapidly tunes the ground state chromophore equilibrium from the phenolate to the phenol state generating a pH-dependent, turn-off fluorescence response. Furthermore, live-cell fluorescence microscopy reveals that GFPxm163 provides a reversible, yet indirect readout of chloride transport via iodide exchange. With this demonstration, we anticipate that the pairing of bioinformatics with protein engineering methods will provide an efficient methodology to discover and design new chloride-sensitive fluorescent proteins for cellular applications.

Massively parallel characterization of CYP2C9 variant enzyme activity and abundance

CYP2C9 encodes a cytochrome P450 enzyme responsible for metabolizing up to 15% of small molecule drugs, and CYP2C9 variants can alter the safety and efficacy of these therapeutics. In particular, the anti-coagulant warfarin is prescribed to over 15 million people annually and polymorphisms in CYP2C9 can affect individual drug response and lead to an increased risk of hemorrhage. We developed click-seq, a pooled yeast-based activity assay, to test thousands of variants. Using click-seq, we measured the activity of 6,142 missense variants in yeast. We also measured the steady-state cellular abundance of 6,370 missense variants in a human cell line by using variant abundance by massively parallel sequencing (VAMP-seq). These data revealed that almost two-thirds of CYP2C9 variants showed decreased activity and that protein abundance accounted for half of the variation in CYP2C9 function. We also measured activity scores for 319 previously unannotated human variants, many of which may have clinical relevance.

Dissecting the Structural Plasticity and Dynamics of Cytochrome P450 2B4 by Molecular Dynamics Simulations

The plasticity of cytochromes P450 (P450s) is known to contribute significantly to their catalytic capacity of metabolizing various substrates. Although numerous studies have been performed, factors governing the plasticity and dynamics of P450s are still not fully understood. In this study, taking CYP2B4 as an example, we dissect the protein plasticity and dynamics in different environments. CYP2B4 is featured by a high degree of plasticity, which exhibits open, closed, and intermediate states. By analyzing the CYP2B4 crystal structures, we identified the structural features for the closed, open, and intermediate states. Interestingly, formation of the dimer structure was found in the open and intermediate states. The subsequent molecular dynamics (MD) simulations of the open structure in water confirmed the importance of the dimer form in stabilizing the open conformations. MD simulations of the closed and open structures in the membrane environment and the free energies for opening the F-G cassette obtained from the umbrella sampling calculations indicate that the membrane environment is important for stabilizing the F-G cassette. The dynamical network analysis indicates that Asp105 on the B-C loop plays an important role in transiting the structure from the open to the intermediate state. Our results thus unveil the mechanisms of dimer formation and open-to-intermediate transition for CYP2B4 in the water and membrane environments.

HaloTag-Based Hybrid Targetable and Ratiometric Sensors for Intracellular Zinc

We report a new series of small molecule-protein hybrid zinc sensors that combine genetic targetability with the spectroscopic profile of synthetic fluorophores. We functionalized the zinc sensor ZinPyr-1 (ZP1) with a chloroalkane linker (ZP1-12Cl) that reacts specifically with the engineered protein HaloTag. The resulting construct, ZP1-HaloTag, binds zinc ions with a threefold fluorescence enhancement. Through exploitation of the protein synthesis machinery of live cells, the HaloTag protein component was expressed, and the ZP1-HaloTag hybrid was assembled upon bath application of ZP1-12Cl. After fusion of HaloTag with targeting peptides or proteins, the resulting hybrid sensor could be directed to specific subcellular locales, including the nucleus, mitochondrial outer membrane, and endoplasmic reticulum. Furthermore, HaloTag was linked with the red fluorescent protein mCherry, permitting formation of a two-fluorophore system that provides not only targetable but also ratiometric sensing of cellular zinc. This system reversibly detects both exogenous and endogenous mobile Zn2+ in response to reactive nitrogen species in live HeLa cells. HaloTag-based hybrid zinc sensors offer new opportunities for visualizing and quantifying biological mobile zinc at discrete subcellular compartments.

Giant Endoplasmic Reticulum vesicles (GERVs), a novel model membrane tool

Artificial giant vesicles have proven highly useful as membrane models in a large variety of biophysical and biochemical studies. They feature accessibility for manipulation and detection, but lack the compositional complexity needed to reconstitute complicated cellular processes. For the plasma membrane (PM), this gap was bridged by the establishment of giant PM vesicles (GPMVs). These native membranes have facilitated studies of protein and lipid diffusion, protein interactions, electrophysiology, fluorescence analysis of lateral domain formation and protein and lipid partitioning as well as mechanical membrane properties and remodeling. The endoplasmic reticulum (ER) is key to a plethora of biological processes in any eukaryotic cell. However, its intracellular location and dynamic and intricate tubular morphology makes it experimentally even less accessible than the PM. A model membrane, which will allow the afore-mentioned types of studies on GPMVs to be performed on ER membranes outside the cell, is therefore genuinely needed. Here, we introduce the formation of giant ER vesicles, termed GERVs, as a new tool for biochemistry and biophysics. To obtain GERVs, we have isolated ER membranes from Saccharomyces cerevisiae and fused them by exploiting the atlastin-like fusion protein Sey1p. We demonstrate the production of GERVs and their utility for further studies.

Post-Translational Regulation of CYP450s Metabolism As Revealed by All-Atoms Simulations of the Aromatase Enzyme

Phosphorylation by kinases enzymes is a widespread regulatory mechanism able of rapidly altering the function of target proteins. Among these are cytochrome P450s (CYP450), a superfamily of enzymes performing the oxidation of endogenous and exogenous substrates thanks to the electron supply of a redox partner. In spite of its pivotal role, the molecular mechanism by which phosphorylation modulates CYP450s metabolism remains elusive. Here by performing microsecond-long all-atom molecular dynamics simulations, we disclose how phosphorylation regulates estrogen biosynthesis, catalyzed by the Human Aromatase (HA) enzyme. Namely, we unprecedentedly propose that HA phosphorylation at Y361 markedly stabilizes its adduct with the flavin mononucleotide domain of CYP450s reductase (CPR), the redox partner of microsomal CYP450s, and a variety of other proteins. With CPR present at physiological conditions in a limiting ratio with respect to its multiple oxidative partners, the enhanced stability of the CPR/HA adduct may favor HA in the competition with the other proteins requiring CPR's electron supply, ultimately facilitating the electron transfer and estrogen biosynthesis. As a result, our work elucidates at atomic-level the post-translational regulation of CYP450s catalysis. Given the potential for rational clinical management of diseases associated with steroid metabolism disorders, unraveling this mechanism is of utmost importance, and raises the intriguing perspective of exploiting this knowledge to devise novel therapies.

Dissociation Constants of Cytochrome P450 2C9/Cytochrome P450 Reductase Complexes in a Lipid Bilayer Membrane Depend on NADPH: A Single-Protein Tracking Study

Cytochrome P450-reductase (CPR) is a versatile NADPH-dependent electron donor located in the cytoplasmic side of the endoplasmic reticulum. It is an electron transferase that is able to deliver electrons to a variety of membrane-bound oxidative partners, including the drug-metabolizing enzymes of the cytochrome P450s (P450). CPR is also stoichiometrically limited compared to its oxidative counterparts, and hypotheses have arisen about possible models that can overcome the stoichiometric imbalance, including quaternary organization of P450 and diffusion-limited models. Described here are results from a single-protein tracking study of fluorescently labeled CPR and cytochrome P450 2C9 (CYP2C9) molecules in which stochastic analysis was used to determine the dissociation constants of CPR/CYP2C9 complexes in a lipid bilayer membrane for the first time. Single-protein trajectories demonstrate the transient nature of these CPR-CYP2C9 interactions, and the measured K_d values are highly dependent on the redox state of CPR. It is shown that CPR^ox/CYP2C9 complexes have a much higher dissociation constant than CPR^2-/CYP2C9 or CPR^4-/CYP2C9 complexes, and a model is presented to account for these results. An Arrhenius analysis of diffusion constants was also carried out, demonstrating that the reduced forms of CPR and CYP2C9 interact differently with the biomimetic ER and may, in addition to protein conformational changes, contribute to the observed NADPH-dependent shift in K_d. Finally, it is also shown that the CPR^ox/CYP2C9 affinity depends on the nature of the ligand, being higher when a substrate is bound, compared to an inhibitor.

The functional effects of physical interactions involving cytochromes P450: putative mechanisms of action and the extent of these effects in biological membranes

Cytochromes P450 represent a family of enzymes, which are responsible for the oxidative metabolism of a wide variety of xenobiotics. Although the mammalian P450s require interactions with their redox partners in order to function, more recently, P450 system proteins have been shown to exist as multi-protein complexes that include the formation of P450•P450 complexes. Evidence has shown that the metabolism of some substrates by a given P450 can be influenced by the specific interaction of the enzyme with other forms of P450. Detailed kinetic analysis of these reactions in vitro has shown that the P450-P450 interactions can alter metabolism by changing the ability of a P450 to bind to its cognate redox partner, NADPH-cytochrome P450 reductase; by altering substrate binding to the affected P450; and/or by changing the rate of a catalytic step of the reaction cycle. This review summarizes the known examples of P450-P450 interactions that have been shown in vitro to influence metabolism and categorizes them according to the mechanism(s) causing the effects. P450-P450 interactions have the potential to cause major changes in the metabolism and elimination of drugs in vivo. This review summarizes the evidence that the P450-P450 interactions influence metabolism in biological membranes and discusses the studies, which will provide further insight into the extent of these effects in the future.

Getting membrane proteins on and off the shuttle bus between the endoplasmic reticulum and the Golgi complex

Secretory proteins exit the endoplasmic reticulum (ER) in coat protein complex II (COPII)-coated vesicles and then progress through the Golgi complex before delivery to their final destination. Soluble cargo can be recruited to ER exit sites by signal-mediated processes (cargo capture) or by bulk flow. For membrane proteins, a third mechanism, based on the interaction of their transmembrane domain (TMD) with lipid microdomains, must also be considered. In this Commentary, I review evidence in favor of the idea that partitioning of TMDs into bilayer domains that are endowed with distinct physico-chemical properties plays a pivotal role in the transport of membrane proteins within the early secretory pathway. The combination of such self-organizational phenomena with canonical intermolecular interactions is most likely to control the release of membrane proteins from the ER into the secretory pathway.

Directed evolution of APEX2 for electron microscopy and proximity labeling

APEX is an engineered peroxidase that functions as an electron microscopy tag and a promiscuous labeling enzyme for live-cell proteomics. Because limited sensitivity precludes applications requiring low APEX expression, we used yeast-display evolution to improve its catalytic efficiency. APEX2 is far more active in cells, enabling the use of electron microscopy to resolve the submitochondrial localization of calcium uptake regulatory protein MICU1. APEX2 also permits superior enrichment of endogenous mitochondrial and endoplasmic reticulum membrane proteins.

Calmodulin-controlled spatial decoding of oscillatory Ca2+ signals by calcineurin

Calcineurin is responsible for mediating a wide variety of cellular processes in response to dynamic calcium (Ca(2+)) signals, yet the precise mechanisms involved in the spatiotemporal control of calcineurin signaling are poorly understood. Here, we use genetically encoded fluorescent biosensors to directly probe the role of cytosolic Ca(2+) oscillations in modulating calcineurin activity dynamics in insulin-secreting MIN6 β-cells. We show that Ca(2+) oscillations induce distinct temporal patterns of calcineurin activity in the cytosol and plasma membrane vs at the ER and mitochondria in these cells. Furthermore, we found that these differential calcineurin activity patterns are determined by variations in the subcellular distribution of calmodulin (CaM), indicating that CaM plays an active role in shaping both the spatial and temporal aspects of calcineurin signaling. Together, our findings provide new insights into the mechanisms by which oscillatory signals are decoded to generate specific functional outputs within different cellular compartments.

Brassinosteroids regulate plant growth through distinct signaling pathways in Selaginella and Arabidopsis

Brassinosteroids (BRs) are growth-promoting steroid hormones that regulate diverse physiological processes in plants. Most BR biosynthetic enzymes belong to the cytochrome P450 (CYP) family. The gene encoding the ultimate step of BR biosynthesis in Arabidopsis likely evolved by gene duplication followed by functional specialization in a dicotyledonous plant-specific manner. To gain insight into the evolution of BRs, we performed a genomic reconstitution of Arabidopsis BR biosynthetic genes in an ancestral vascular plant, the lycophyte Selaginella moellendorffii. Selaginella contains four members of the CYP90 family that cluster together in the CYP85 clan. Similar to known BR biosynthetic genes, the Selaginella CYP90s exhibit eight or ten exons and Selaginella produces a putative BR biosynthetic intermediate. Therefore, we hypothesized that Selaginella CYP90 genes encode BR biosynthetic enzymes. In contrast to typical CYPs in Arabidopsis, Selaginella CYP90E2 and CYP90F1 do not possess amino-terminal signal peptides, suggesting that they do not localize to the endoplasmic reticulum. In addition, one of the three putative CYP reductases (CPRs) that is required for CYP enzyme function co-localized with CYP90E2 and CYP90F1. Treatments with a BR biosynthetic inhibitor, propiconazole, and epi-brassinolide resulted in greatly retarded and increased growth, respectively. This suggests that BRs promote growth in Selaginella, as they do in Arabidopsis. However, BR signaling occurs through different pathways than in Arabidopsis. A sequence homologous to the Arabidopsis BR receptor BRI1 was absent in Selaginella, but downstream components, including BIN2, BSU1, and BZR1, were present. Thus, the mechanism that initiates BR signaling in Selaginella seems to differ from that in Arabidopsis. Our findings suggest that the basic physiological roles of BRs as growth-promoting hormones are conserved in both lycophytes and Arabidopsis; however, different BR molecules and BRI1-based membrane receptor complexes evolved in these plants.

Protein-protein and protein-membrane associations in the lignin pathway

Supramolecular organization of enzymes is proposed to orchestrate metabolic complexity and help channel intermediates in different pathways. Phenylpropanoid metabolism has to direct up to 30% of the carbon fixed by plants to the biosynthesis of lignin precursors. Effective coupling of the enzymes in the pathway thus seems to be required. Subcellular localization, mobility, protein-protein, and protein-membrane interactions of four consecutive enzymes around the main branch point leading to lignin precursors was investigated in leaf tissues of Nicotiana benthamiana and cells of Arabidopsis thaliana. CYP73A5 and CYP98A3, the two Arabidopsis cytochrome P450s (P450s) catalyzing para- and meta-hydroxylations of the phenolic ring of monolignols were found to colocalize in the endoplasmic reticulum (ER) and to form homo- and heteromers. They moved along with the fast remodeling plant ER, but their lateral diffusion on the ER surface was restricted, likely due to association with other ER proteins. The connecting soluble enzyme hydroxycinnamoyltransferase (HCT), was found partially associated with the ER. Both HCT and the 4-coumaroyl-CoA ligase relocalized closer to the membrane upon P450 expression. Fluorescence lifetime imaging microscopy supports P450 colocalization and interaction with the soluble proteins, enhanced by the expression of the partner proteins. Protein relocalization was further enhanced in tissues undergoing wound repair. CYP98A3 was the most effective in driving protein association.

Assessing the tendency of fluorescent proteins to oligomerize under physiologic conditions

Several fluorescent proteins (FPs) are prone to forming low-affinity oligomers. This undesirable tendency is exacerbated when FPs are confined to membranes or when fused to naturally oligomeric proteins. Oligomerization of FPs limits their suitability for creating fusions with proteins of interest. Unfortunately, no standardized method evaluates the biologically relevant oligomeric state of FPs. Here, we describe a quantitative visual assay for assessing whether FPs are sufficiently monomeric under physiologic conditions. Membrane-associated FP-fusion proteins, by virtue of their constrained planar geometry, achieve high effective concentrations. We exploited this propensity to develop an assay to measure FP tendencies to oligomerize in cells. FPs were fused on the cytoplasmic end of an endoplasmic reticulum (ER) signal-anchor membrane protein (CytERM) and expressed in cells. Cells were scored based on the ability of CytERM to homo-oligomerize with proteins on apposing membranes and restructure the ER from a tubular network into organized smooth ER (OSER) whorl structures. The ratio of nuclear envelope and OSER structures mean fluorescent intensities for cells expressing enhanced green fluorescent protein (EGFP) or monomeric green fluorescent protein (mGFP) CytERM established standards for comparison of uncharacterized FPs. We tested three FPs and identified two as sufficiently monomeric, while a third previously reported as monomeric was found to strongly oligomerize.

Membrane protein complexes catalyze both 4- and 3-hydroxylation of cinnamic acid derivatives in monolignol biosynthesis

The hydroxylation of 4- and 3-ring carbons of cinnamic acid derivatives during monolignol biosynthesis are key steps that determine the structure and properties of lignin. Individual enzymes have been thought to catalyze these reactions. In stem differentiating xylem (SDX) of Populus trichocarpa, two cinnamic acid 4-hydroxylases (PtrC4H1 and PtrC4H2) and a p-coumaroyl ester 3-hydroxylase (PtrC3H3) are the enzymes involved in these reactions. Here we present evidence that these hydroxylases interact, forming heterodimeric (PtrC4H1/C4H2, PtrC4H1/C3H3, and PtrC4H2/C3H3) and heterotrimeric (PtrC4H1/C4H2/C3H3) membrane protein complexes. Enzyme kinetics using yeast recombinant proteins demonstrated that the enzymatic efficiency (V(max)/k(m)) for any of the complexes is 70-6,500 times greater than that of the individual proteins. The highest increase in efficiency was found for the PtrC4H1/C4H2/C3H3-mediated p-coumaroyl ester 3-hydroxylation. Affinity purification-quantitative mass spectrometry, bimolecular fluorescence complementation, chemical cross-linking, and reciprocal coimmunoprecipitation provide further evidence for these multiprotein complexes. The activities of the recombinant and SDX plant proteins demonstrate two protein-complex-mediated 3-hydroxylation paths in monolignol biosynthesis in P. trichocarpa SDX; one converts p-coumaric acid to caffeic acid and the other converts p-coumaroyl shikimic acid to caffeoyl shikimic acid. Cinnamic acid 4-hydroxylation is also mediated by the same protein complexes. These results provide direct evidence for functional involvement of membrane protein complexes in monolignol biosynthesis.

The signal-anchor sequence of CYP2C1 inserts into the membrane as a hairpin structure

Microsomal cytochrome P450s (CYPs) are anchored to the endoplasmic reticulum membrane by the N-terminal signal-anchor sequence which is predicted to insert into the membrane as a type 1 transmembrane helix with a luminally located N-terminus. We have mapped amino acids of the CYP2C1 signal-anchor, fused to Cys-free glutathione S-transferase, within the membrane by Cys-specific labeling with membrane-impermeant maleimide polyethylene glycol. At the C-terminal end of the signal-anchor, Trp-20 was mapped to the membrane-cytosol interface and Leu-19 was within the membrane. Unexpectedly, at the N-terminal end, Glu-2 and Pro-3 were mapped to the cytoplasmic side of the membrane rather than the luminal side as expected of a type 1 transmembrane helix. Similar results were observed for the N-terminal amino acids of the signal-anchor sequences of CYP3A4 and CYP2E1. These observations indicate that contrary to the current model of the signal-anchor of CYPs as a type 1 transmembrane helix, CYP2C1, CYP2E1, and CYP3A4 are monotopic membrane proteins with N-terminal signal-anchors that have a hairpin or wedge orientation in the membrane.

Protein kinase A activity at the endoplasmic reticulum surface is responsible for augmentation of human ether-a-go-go-related gene product (HERG)

Human ether-a-go-go-related gene product (HERG) is a cardiac potassium channel commonly implicated in the pathogenesis of the long QT syndrome, type 2 (LQT2). LQT2 mutations typically have incomplete penetrance and affect individuals at various stages of their lives; this may mirror variations in intracellular signaling and HERG regulation. Previous work showed that sustained protein kinase A (PKA) activity augments HERG protein abundance by a mechanism that includes enhanced protein translation. To investigate the subcellular site of this regulation, we generated site-specific probes to the cytoplasmic surface of the endoplasmic reticulum (ER), the presumed locale of channel synthesis. Real-time FRET-based indicators demonstrated both cAMP and PKA activity at the ER. A PKA inhibitor targeted to the ER surface (termed p4PKIg) completely abolished PKA-mediated augmentation of HERG in HEK293 cells as well as rat neonatal cardiomyocytes. Immunofluorescence co-localization, targeted FRET-based PKA biosensors, phospho-specific antibodies, and in vivo phosphorylation experiments confirmed that p4PKIg is preferentially active at the ER surface rather than the plasma membrane. Rerouting this inhibitor to the outer mitochondrial membrane diminishes its ability to block cAMP-dependent HERG induction. Our results support a model where PKA-dependent regulation of HERG synthesis occurs at the ER surface. Furthermore, reagents generated for this study provide novel experimental tools to probe compartmentalized cAMP/PKA signaling within cells.

Progesterone receptor membrane component 1 inhibits the activity of drug-metabolizing cytochromes P450 and binds to cytochrome P450 reductase

Progesterone receptor membrane component 1 (PGRMC1) has been shown to interact with several cytochromes P450 (P450s) and to activate enzymatic activity of P450s involved in sterol biosynthesis. We analyzed the interactions of PGRMC1 with the drug-metabolizing P450s, CYP2C2, CYP2C8, and CYP3A4, in transfected cells. Based on coimmunoprecipitation assays, PGRMC1 bound efficiently to all three P450s, and binding to the catalytic cytoplasmic domain of CYP2C2 was much more efficient than to a chimera containing only the N-terminal transmembrane domain. Down-regulation of PGRMC1 expression levels in human embryonic kidney 293 and HepG2 cell lines stably expressing PGRMC1-specific small interfering RNA had no effect on the endoplasmic reticulum localization and expression levels of P450s, whereas enzymatic activities of CYP2C2, CYP2C8, and CYP3A4 were slightly higher in PGRMC1-deficient cells. Cotransfection of cells with P450s and PGRMC1 resulted in PGRMC1 concentration-dependent inhibition of the P450 activities, and this inhibition was partially reversed by increased expression of the P450 reductase (CPR). In contrast, CYP51 activity was decreased by down-regulation of PGRMC1 and expression of PGRMC1 in the PGRMC1-deficient cells increased CYP51 activity. In cells cotransfected with CPR and PGRMC1, strong binding of CPR to PGRMC1 was observed; however, in the presence of CYP2C2, interaction of PGRMC1 with CPR was significantly reduced, suggesting that CYP2C2 competes with CPR for binding to PGRMC1. These data show that in contrast to sterol synthesizing P450, PGRMC1 is not required for the activities of several drug-metabolizing P450s, and its overexpression inhibits those P450 activities. Furthermore, PGRMC1 binds to CPR, which may influence P450 activity.

Colorectal cancer-specific cytochrome P450 2W1: intracellular localization, glycosylation, and catalytic activity

Cytochrome P450 2W1 (CYP2W1) is expressed at high levels in colorectal cancer cells. Moreover, we have shown previously that a higher tumor expression is associated with less survival. In this study, we characterize post-translational modification, inverted endoplasmic reticulum (ER) topology, and catalytic activity of CYP2W1. The analysis of colorectal normal and cancer tissues and CYP2W1 overexpressing human embryonic kidney (HEK) 293 cells showed that a fraction of CYP2W1 is modified by N-glycosylation. Bioinformatic analysis identified Asn177 as the only possible glycosylation site of CYP2W1, which was supported by the inability of an N177A mutant to be glycosylated in HEK 293 cells. Analysis of the membrane topology indicated that unlike other cytochromes P450, CYP2W1 in HEK 293-transfected cells and in nontransfected Caco2TC7 and HepG2 cells is oriented toward the lumen of the ER, a topology making CYP2W1 available to the ER glycosylation machinery. Immunofluorescence microscopy and cell surface biotinylation experiments revealed approximately 8% of the CYP2W1 on the cell surface. Despite the reverse orientation of CYP2W1 in the ER membrane, apparently making functional interactions with NADPH-cytochrome P450 reductase impossible, CYP2W1 in HEK 293 cells was active in the metabolism of indoline substrates and was able to activate aflatoxin B1 into cytotoxic products. The study identifies for the first time a cytochrome P450 enzyme with a luminal ER orientation and still retaining catalytic activity. Together, these results suggest the possibility of using CYP2W1 as a drug target in the treatment of colon cancer using antibodies and/or specific CYP2W1 activated prodrugs.

Proteasome inhibition compromises direct retention of cytochrome P450 2C2 in the endoplasmic reticulum

To determine whether protein degradation plays a role in the endoplasmic reticulum (ER) retention of cytochromes P450, the effects of proteasomal inhibitors on the expression and distribution of green fluorescent protein chimeras of CYP2C2 and related proteins was examined. In transfected cells, expression levels of chimeras of full-length CYP2C2 and its cytosolic domain, but not its N-terminal transmembrane sequence, were increased by proteasomal inhibition. Redistribution of all three chimeras from the reticular ER into a perinuclear compartment and, in a subset of cells, also to the cell surface was observed after proteasomal inhibition. Redistribution was blocked by the microtubular inhibitor, nocodazole, suggesting that redistribution to the cell surface followed the conventional vesicular transport pathway. Similar redistributions were detected for BAP31, a CYP2C2 binding chaperone; CYP2E1 and CYP3A4, which are also degraded by the proteasomal pathway; and for cytochrome P450 reductase, which does not undergo proteasomal degradation; but not for the ER membrane proteins, sec61 and calnexin. Redistribution does not result from saturation of an ER retention "receptor" since in some cases protein levels were unaffected. Proteasomal inhibition may, therefore, alter ER retention by affecting a protein critical for ER retention, either directly, or indirectly by affecting the composition of the ER membranes.

Influence of protein-protein interactions on the cellular localization of cytochrome P450

BACKGROUND

Microsomal CYPs are integral membrane proteins that are localized in the endoplasmic reticulum (ER), which is critical for their function. CYPs are co-translationally inserted into the rough ER membrane and are then either directly retained in the smooth ER or retained by a retrieval mechanism or targeted for ER-associated degradation. Protein-protein interactions are likely to be important for proper cellular targeting of CYPs.

OBJECTIVE

Progress in understanding the mechanisms of cellular targeting and ER retention of CYPs is reviewed with emphasis on the role of protein-protein interactions. Possible mechanisms of direct retention are the incorporation of CYPs into an immobile complex in the ER membrane, homooligomerization that prevents inclusion in transport vesicles, exclusion of CYP monomers from transport vesicles or targeting of CYPs to an ER subdomain away from sites of transport vesicle formation. Degradation of CYPs occurs either by lysosomal mechanisms or by the ubiquitin-proteasomal pathway.

METHODS

The scope of this review includes studies published in the research literature that have defined the targeting of CYPs to the ER, the retention of CYPs in the ER and the degradation of CYPs.

RESULTS/CONCLUSION

Targeting of CYPs to the ER is well understood and involves signal recognition particle-mediated delivery to the sec61 complex. The mechanism of ER retention of CYPs remains unclear, but self-oligomerization or binding to large immobile networks do not underlie ER retention of CYPs. An ER retention 'receptor' remains elusive, but BAP31 is important for the proper cellular localization of CYPs and Dap1p is a CYP-binding protein that is a candidate for such a receptor. Identification of protein binding partners of CYPs will be critical to understanding the mechanism of ER retention.

Kinetics of bimolecular reactions in model bilayers and biological membranes. A critical review

The quantitative study of the probability of molecular encounters giving rise to a reaction in membranes is a challenging discipline. Model systems, model in the sense that they use model bilayers and model reactants, have been widely used for this purpose, but the methodologies employed for the analysis of the results obtained in experiments, and for experimental design, are so disparate that a concerned experimentalist has difficulty in deciding about the value of each approach. This review intends to examine the several approaches that can be found in the literature showing, when feasible, the weakness, strengths and limits of application of each of them. There is not, so far, a full experimental validation of the most promising theories for the analysis of reactions in two dimensions, what leaves open a large field for new research. The major challenge resides in the time range in which the processes take place, but the possibilities of the existing techniques for these studies are far from exhausted. We review also the attempts of several authors to quantitatively analyze the kinetics of reactions in biological membranes. Especially in this field, the recently developed microspectroscopies enclose a still unexplored potential.

Agonist-evoked inositol trisphosphate receptor (IP3R) clustering is not dependent on changes in the structure of the endoplasmic reticulum

The size and number of IP3R (inositol 1,4,5-trisphosphate receptor) clusters located on the surface of the ER (endoplasmic reticulum) is hypothesized to regulate the propagation of Ca2+ waves in cells, but the mechanisms by which the receptors cluster are not understood. Using immunocytochemistry, live-cell imaging and heterologous expression of ER membrane proteins we have investigated IP3R clustering in the basophilic cell line RBL-2H3 following the activation of native cell-surface antigen receptors. IP3R clusters are present in resting cells, and upon receptor stimulation, form larger aggregates. Cluster formation and maintenance required the presence of extracellular Ca2+ in both resting and stimulated cells. Using transfection with a marker of the ER, we found that the ER itself also showed structural changes, leading to an increased number of 'hotspots', following antigen stimulation. Surprisingly, however, when we compared the ER hotspots and IP3R clusters, we found them to be distinct. Imaging of YFP (yellow fluorescent protein)-IP3R transfected in to living cells confirmed that IP3R clustering increased upon stimulation. Photobleaching experiments showed that the IP3R occupied a single contiguous ER compartment both before and after stimulation, suggesting a dynamic exchange of IP3R molecules between the clusters and the surrounding ER membrane. It also showed a decrease in the mobile fraction after cell activation, consistent with receptor anchoring within clusters. We conclude that IP3R clustering in RBL-2H3 cells is not simply a reflection of bulk-changes in ER structure, but rather is due to the receptor undergoing homotypic or heterotypic protein-protein interactions in response to agonist stimulation.

BAP31 Is Involved in the Retention of Cytochrome P450 2C2 in the Endoplasmic Reticulum

Microsomal cytochrome P450 2C2 is an integral endoplasmic reticulum (ER) membrane protein that is directly retained in the ER and excluded from transport vesicles. We have used bimolecular fluorescence complementation and co-immunoprecipitation to show that a ubiquitous ER membrane protein (BAP31) interacts with P450 2C2 in transfected COS-1 cells. A chimera containing only the N-terminal signal anchor of P450 2C1 (P450 2C1-(1-29)) also interacted with BAP31, which is consistent with interaction of the two proteins via their transmembrane domains. Down-regulation of BAP31 expression with small interfering RNA resulted in redistribution of green fluorescent protein-tagged P450 2C2 or P450 2C1-(1-29) from the ER into the nuclear membrane and compact perinuclear compartment structures as well as the cell surface in a small fraction of the cells. In Bap31-null embryonic stem cells, a significant fraction of P450 2C2 or P450 2C1-(1-29) was detected at the cell surface and nuclear envelope, but was redistributed to the ER by expression of BAP31. The expression level of P450 2C2 was significantly increased in COS-1 cells with repressed levels of BAP31. Formation of the pro-apoptotic p20 fragment of BAP31 was detected in transfected COS-1 cells expressing P450 2C2, and annexin V staining was consistent with the activation of an apoptotic pathway in these cells. Down-regulation of BAP31 with small interfering RNA partially reversed the apoptosis. These results suggest that interaction of P450 2C2 with BAP31 is important for its ER retention and expression level and that BAP31 may be involved in the regulation of apoptosis induced by the ER overload response to increased expression of P450.

Dynamics of the Ins(1,4,5)P3 receptor during polarization of MDCK cells

BACKGROUND INFORMATION

The uneven distribution of the Ins(1,4,5)P3R [Ins(1,4,5)P3 receptor] within the ER (endoplasmic reticulum) membrane generates spatially complex Ca2+ signals. The ER is a dynamic network, which allows the rapid diffusion of membrane proteins from one part of the cell to another. However, little is known about the localization and the dynamics of the Ins(1,4,5)P3R in the ER of living cells. We have used a MDCK (Madin-Darby canine kidney) clone stably expressing the Ins(1,4,5)P3R1-GFP (where GFP stands for green fluorescent protein) to investigate the effect of cell polarity on the lateral mobility of the Ins(1,4,5)P3R.

RESULTS

In non-confluent MDCK cells, the chimaera is homogeneously distributed throughout the ER and the nuclear envelope. FRAP (fluorescence recovery after photobleaching) experiments showed that the receptor can move freely in the ER with a diffusion constant (D=0.01 microm2/s) approx. ten times lower than other ER membrane proteins. In confluent polarized cells, two populations of receptor can be defined: one population is distributed in the cytoplasm and is mobile but with a slower diffusion constant (D=0.004 microm2/s) compared with non-confluent cells, whereas the other population is concentrated at the periphery of the cells and is apparently immobile.

CONCLUSIONS

The observed differences in the mobility of the Ins(1,4,5)P3R are most probably due to its interactions with stable protein complexes that form at the periphery of the polarized cells.

FMDV–2A sequence and protein arrangement contribute to functionality of CYP2B1–reporter fusion protein

Two optimized forms of green fluorescence proteins (GFP), enhanced GFP (EGFP) and humanized Renilla GFP (hrGFP), were used to track expression of cytochrome P450 2B1 (CYP2B1), an endoplasmic reticulum membrane-bound protein. In transiently expressing HEK293 cells we show that CYP2B1-GFP fusion proteins are stable and functional, whereas the vice-versa-arranged GFP-CYP2B1 fusions are not. The CYP2B1-hrGFP fusion protein is characterized by reduction in mean fluorescence intensity (MFI) to less than 20% of that of the hrGFP protein alone, accompanied by a 50% loss of CYP2B1 activity. Exchanging the linker for an alpha-helical peptide structure between CYP2B1 and hrGFP does not improve fusion protein activity. Insertion of a short linker (five amino acids) increases reporter protein fluorescence intensity twofold without improving CYP2B1 activity. Introduction of the foot and mouth disease virus 2A sequence providing cotranslational cleavage led to an unstable hrGFP-2A protein, whereas the corresponding EGFP-2A protein was stable and yielded an MFI superior to those of all other fusion constructs tested. CYP2B1 activity of the EGFP-2A-CYP2B1 protein was in the range of that of the unmodified CYP2B1. These data indicate that the protein arrangement EGFP-2A-CYP2B1 is superior to others, since it is most active and visible, which is essential for an effective tracking of the CYP2B1 enzyme.

BIMOLECULAR FLUORESCENCE COMPLEMENTATION ANALYSIS OF CYTOCHROME P450 2C2, 2E1, AND NADPH-CYTOCHROME P450 REDUCTASE MOLECULAR INTERACTIONS IN LIVING CELLS

Interactions between cytochromes P450 (P450s) and P450 reductase are required for enzymatic activity, and homo- or heterooligomerization of P450s may also be functionally important. Bimolecular fluorescence complementation (BiFC) was used to examine P450 interactions in a natural membrane context within living cells. BiFC detects protein interactions in living cells by reconstitution of a fluorescent protein from two fragments that are fused to the two interacting proteins. Nonspecific protein-protein interactions were detected if proteins were expressed at high levels. At low protein expression levels, homo-oligomerization of P450 2C2, but not P450 2E1, and interactions of these P450s with P450 reductase were detected by BiFC, consistent with interactions detected previously by fluorescence resonance emission transfer. Weak interaction of P450 2C2 with P450 2E1 and homooligomerization of P450 reductase was also detected by BiFC. Homo-oligomerization of the N-terminal P450 2C1 signal anchor sequence and interactions between the signal anchor and full-length P450 2C2 were detected, suggesting that homo-oligomerization of P450 2C2 is mediated by the signal anchor. However, interactions between the signal anchor and either P450 2E1 or P450 reductase were not detected by BiFC. Although high concentrations of the substrate lauric acid increased BiFC for both P450 2E1 and P450 2C2 with P450 reductase, the concentration dependence did not correlate with reported K(m) values. These results demonstrate that BiFC is an effective method to study the complex protein interactions that occur within the microsomal P450 system in living cells.

Endoplasmic Reticulum Localization of Gaa1 and PIG-T, Subunits of the Glycosylphosphatidylinositol Transamidase Complex

After integration into the endoplasmic reticulum (ER) membrane, ER-resident membrane proteins must be segregated from proteins that are exported to post-ER compartments. Here we analyze how human Gaa1 and PIG-T, two of the five subunits of the ER-localized glycosylphosphatidylinositol transamidase complex, are retained in the ER. Neither protein contains a known ER localization signal. Gaa1 is a polytopic membrane glycoprotein with a cytoplasmic N terminus and a large luminal loop between its first two transmembrane spans; PIG-T is a type I membrane glycoprotein. To simplify our analyses, we studied Gaa1 and PIG-T constructs that could not interact with other subunits of the transamidase. We now show that Gaa1(282), a truncated protein consisting of the first TM domain and luminal loop of Gaa1, is correctly oriented, N-glycosylated, and ER-localized. Removal of a potential ER localization signal in the form of a triple arginine cluster near the N terminus of Gaa1 or Gaa1(282) had no effect on ER localization. Fusion proteins consisting of different elements of Gaa1(282) appended to alpha2,6-sialyltransferase or transferrin receptor could exit the ER, indicating that Gaa1(282), and by implication Gaa1, does not contain any dominant ER-sorting determinants. The data suggest that Gaa1 is passively retained in the ER by a signalless mechanism. In contrast, similar analyses of PIG-T revealed that it is ER-localized because of information in its transmembrane span; fusion of the PIG-T transmembrane span to Tac antigen, a plasma membrane-localized protein, caused the fusion protein to remain in the ER. These data are discussed in the context of models that have been proposed to account for retention of ER membrane proteins.

Derivation of a closed form analytical expression for fluorescence recovery after photo bleaching in the case of continuous bleaching during read out

Measurements of very slow diffusive processes in membranes, like the diffusion of integral membrane proteins, by fluorescence recovery after photo bleaching (FRAP) are hampered by bleaching of the probe during the read out of the fluorescence recovery. In the limit of long observation time (very slow diffusion as in the case of large membrane proteins), this bleaching may cause errors to the recovery function and thus provides error-prone diffusion coefficients. In this work we present a new approach to a two-dimensional closed form analytical solution of the reaction-diffusion equation, based on the addition of a dissipative term to the conventional diffusion equation. The calculation was done assuming (i) a Gaussian laser beam profile for bleaching the spot and (ii) that the fluorescence intensity profile emerging from the spot can be approximated by a two-dimensional Gaussian. The detection scheme derived from the analytical solution allows for diffusion measurements without the constraint of observation bleaching. Recovery curves of experimental FRAP data obtained under non-negligible read-out bleaching for native membranes (rabbit endoplasmic reticulum) on a planar solid support showed excellent agreement with the analytical solution and allowed the calculation of the lipid diffusion coefficient.

Translational mobility of the type 3 inositol 1,4,5-trisphosphate receptor Ca2+ release channel in endoplasmic reticulum membrane

The inositol 1,4,5-trisphosphate receptor (InsP3R) is an integral membrane protein in the endoplasmic reticulum (ER) which functions as a ligand-gated Ca2+ release channel. InsP3-mediated Ca2+ release modulates the cytoplasmic free Ca2+ concentration ([Ca2+]i), providing a ubiquitous intracellular signal with high temporal and spatial specificity. Precise localization of the InsP3R is believed to be important for providing local [Ca2+] regulation and for ensuring efficient functional coupling between Ca2+ release sites by enabling graded recruitment of channels with increasing stimulus strength in the face of the intrinsically unstable regenerative process of Ca2+-induced Ca2+ release. Highly localized Ca2+ release has been attributed to the ability of the InsP3R channels to cluster and to be localized to discrete areas, suggesting that mechanisms may exist to restrict their movement. Here, we examined the lateral mobility of the type 3 isoform of the InsP3R (InsP3R3) in the ER membrane by performing confocal fluorescence recovery after photobleaching of an InsP3R3 with green fluorescent protein fused to its N terminus. In Chinese hamster ovary and COS-7 cells, the diffusion coefficient D was approximately 4 x 10(-10) cm2/s at room temperature, a value similar to that determined for other ER-localized integral membrane proteins, with a high fraction (approximately 75%) of channels mobile. D was modestly increased at 37 degrees C, and it as well as the mobile fraction were reversibly reduced by ATP depletion. Although disruption of the actin cytoskeleton (latrunculin) was without effect, disruption of microtubules (nocodazole) reduced D by half without affecting the mobile fraction. We conclude that the entire ER is continuous in these cells, with the large majority of InsP3R3 channels free to diffuse throughout it, at rates that are comparable with those measured for other polytopic ER integral membrane proteins. The observed InsP3R3 mobility may be higher than its intrinsic diffusional mobility because of additional ATP- and microtubule-facilitated motility of the channel.

Gene expression changes associated with the endoplasmic reticulum stress response induced by microsomal cytochrome p450 overproduction

Induction of drug-metabolizing microsomal cytochromes p450 (p450s) results in a striking proliferation of the smooth endoplasmic reticulum (ER). Overexpression of P450s in yeast and cultured cells produces a similar response. The signals mediating this process are not known but probably involve signal transduction pathways involved in the unfolded protein response (UPR) or the ER overload response (EOR). We have examined the temporal response of specific genes in these pathways and genes globally to overexpression of p450 in cultured cells. Activity of NFkappaB, an EOR component, was substantially increased by overexpression of full-length p450 2C2 or a chimera with the 28-amino acid signal anchor sequence of p450 2C2 in HepG2 cells, and the activation correlated temporally with the accumulation of p450 in the cells. In the UPR pathway, activation of the transcription factor XBP1 by IRE1 also correlated with the accumulation of p450 in the cells, and in contrast, maximum activation of the BiP/grp78 promoter preceded the accumulation. Differential effects of expression of p450 on apoptosis were observed in nonhepatic COS1 and hepatic HepG2 cells. In COS1 cells, apoptosis was induced, and this correlated with sustained activation of the pro-apoptotic JNK pathway, induction of CHOP, and an absence of the increased NFkappaB activity. In HepG2 cells, JNK was only transiently activated, and CHOP expression was not induced. As assessed by DNA microarray analysis, up-regulation of signaling genes was predominant including those involved in anti-apoptosis and ER stress. These results suggest that both the EOR and UPR pathways are involved in the cellular response to induction of p450 expression and that in hepatic cells genes are also induced to block apoptosis, which may be a physiologically relevant response to prevent cell death during xenobiotic induced expression of p450 in the liver.

Formation of stacked ER cisternae by low affinity protein interactions

The endoplasmic reticulum (ER) can transform from a network of branching tubules into stacked membrane arrays (termed organized smooth ER [OSER]) in response to elevated levels of specific resident proteins, such as cytochrome b(5). Here, we have tagged OSER-inducing proteins with green fluorescent protein (GFP) to study OSER biogenesis and dynamics in living cells. Overexpression of these proteins induced formation of karmellae, whorls, and crystalloid OSER structures. Photobleaching experiments revealed that OSER-inducing proteins were highly mobile within OSER structures and could exchange between OSER structures and surrounding reticular ER. This indicated that binding interactions between proteins on apposing stacked membranes of OSER structures were not of high affinity. Addition of GFP, which undergoes low affinity, antiparallel dimerization, to the cytoplasmic domains of non–OSER-inducing resident ER proteins was sufficient to induce OSER structures when overexpressed, but addition of a nondimerizing GFP variant was not. These results point to a molecular mechanism for OSER biogenesis that involves weak homotypic interactions between cytoplasmic domains of proteins. This mechanism may underlie the formation of other stacked membrane structures within cells.

Fluorescence resonance energy transfer analysis of cytochromes P450 2C2 and 2E1 molecular interactions in living cells

The molecular organization of microsomal cytochromes P450 (P450s) and formation of complexes with P450 reductase have been studied previously with isolated proteins and in reconstituted systems. Although these studies demonstrated that some P450s oligomerize in vitro, neither oligomerization nor interactions of P450 with P450 reductase have been studied in living cells. Here we have used fluorescence resonance energy transfer (FRET) to study P450 oligomerization and binding to P450 reductase in live transfected cells. Cytochrome P450 2C2, but not P450 2E1, forms homo-oligomeric structures, and this self-association is mediated by the signal-anchor sequence. Because P450 2C2, in contrast to P450 2E1, is directly retained in the endoplasmic reticulum (ER), these results could suggest that oligomerization may prevent transport from the ER. However, P450 2C1 signal-anchor sequence chimera defective in ER retention also formed oligomers, and chimera containing the cytoplasmic domain of P450 2C2, which is directly retained in the ER, did not exhibit self-oligomerization, which indicates that oligomerization is not correlated with direct retention. By using FRET, we have also detected binding of P450 2C2 and P450 2E1 to P450 reductase. In contrast to self-oligomerization, the catalytic domain can mediate an interaction of P450 2C2 with P450 reductase. These results suggest that microsomal P450s may differ in their quaternary structure but that these differences do not detectably affect interaction with the reductase or transport from the ER.

ER retention may play a role in sorting of the nuclear pore membrane protein POM121

Integral membrane proteins of the nuclear envelope (NE) are synthesized on the rough endoplasmic reticulum (ER) and following free diffusion in the continuous ER/NE membrane system are targeted to their proper destinations due to interactions of specific domains with other components of the NE. By studying the intracellular distribution and dynamics of a deletion mutant of an integral membrane protein of the nuclear pores, POM121, which lacks the pore-targeting domain, we investigated if ER retention plays a role in sorting of integral membrane proteins to the nuclear envelope. A nascent membrane protein lacking sorting determinants is believed to diffuse laterally in the continuous ER/NE lipid bilayer and expected to follow vesicular traffic to the plasma membrane. The GFP-tagged deletion mutant, POM121(1-129)-GFP, specifically distributed within the ER membrane, but was completely absent from the Golgi compartment and the plasma membrane. Experiments using fluorescence recovery after photobleaching (FRAP) and fluorescence loss in photobleaching (FLIP) demonstrated that despite having very high mobility within the whole ER network (D = 0.41 +/- 0.11 micro m(2)/s) POM121(1-129)-GFP was unable to exit the ER. It was also not detected in post-ER compartments of cells incubated at 15 degrees C. Taken together, these experiments show that amino acids 1-129 of POM121 are able to retain GFP in the ER membrane and suggest that this retention occurs by a direct mechanism rather than by a retrieval mechanism. Our data suggest that ER retention might be important for sorting of POM121 to the nuclear pores.

Diffusional mobility of the cystic fibrosis transmembrane conductance regulator mutant, delta F508-CFTR, in the endoplasmic reticulum measured by photobleaching of GFP-CFTR chimeras

Mutations in the cystic fibrosis transmembrane conductance regulator protein (CFTR) cause cystic fibrosis. The most common disease-causing mutation, DeltaF508, is retained in the endoplasmic reticulum (ER) and is unable to function as a plasma membrane chloride channel. To investigate whether the ER retention of DeltaF508-CFTR is caused by immobilization and/or aggregation, we have measured the diffusional mobility of green fluorescent protein (GFP) chimeras of wild type (wt)-CFTR and DeltaF508-CFTR by fluorescence recovery after photobleaching. GFP-labeled DeltaF508-CFTR was localized in the ER and wt-CFTR in the plasma membrane and intracellular membranes in transfected COS7 and Chinese hamster ovary K1 cells. Both chimeras localized to the ER after brefeldin A treatment. Spot photobleaching showed that CFTR diffusion (diffusion coefficient approximately 10(-9) cm(2)/s) was not significantly slowed by the DeltaF508 mutation and that nearly all wt-CFTR and DeltaF508-CFTR diffused throughout the ER without restriction. Stabilization of molecular chaperone interactions by ATP depletion produced remarkable DeltaF508-CFTR immobilization ( approximately 50%) and slowed diffusion (6.5 x 10(-10) cm(2)/s) but had little effect on wt-CFTR. Fluorescence depletion experiments revealed that the immobilized DeltaF508-CFTR in ATP-depleted cells remained in an ER pattern. The mobility of wt-CFTR and DeltaF508-CFTR was reduced by maneuvers that alter CFTR processing or interactions with molecular chaperones, including tunicamycin, geldanamycin, and lactacystin. Photobleaching of the fluorescent ER lipid diOC(4)(3) showed that neither ER restructuring nor fragmentation during these maneuvers was responsible for the slowing and immobilization of CFTR. These results suggest that (a) the ER retention of DeltaF508-CFTR is not due to restricted ER mobility, (b) the majority of DeltaF508-CFTR is not aggregated or bound to slowly moving membrane proteins, and (c) DeltaF508-CFTR may interact to a greater extent with molecular chaperones than does wt-CFTR.

The juxtamembrane sequence of cytochrome P-450 2C1 contains an endoplasmic reticulum retention signal

The N-terminal signal anchor of cytochrome P-450 2C1 mediates retention in the endoplasmic reticulum (ER) membrane of several reporter proteins. The same sequence fused to the C terminus of the extracellular domain of the epidermal growth factor receptor permits transport of the chimeric protein to the plasma membrane. In the N-terminal position, the ER retention function of this signal depends on the polarity of the hydrophobic domain and the sequence KQS in the short hydrophilic linker immediately following the transmembrane domain. To determine what properties are required for the ER retention function of the signal anchor in a position other than the N terminus, the effect of mutations in the linker and hydrophobic domains on subcellular localization in COS1 cells of chimeric proteins with the P-450 signal anchor in an internal or C-terminal position was analyzed. For the C-terminal position, the signal anchor was fused to the end of the luminal domain of epidermal growth factor receptor, and green fluorescent protein was additionally fused at the C terminus of the signal anchor for the internal position. In these chimeras, the ER retention function of the signal anchor was rescued by deletion of three leucines at the C-terminal side of its hydrophobic domain; however, deletion of three valines from the N-terminal side did not affect transport to the cell surface. ER retention of the C-terminal deletion mutants was eliminated by substitution of alanines for glutamine and serine in the linker sequence. These data are consistent with a model in which the position of the linker sequence at the membrane surface, which is critical for ER retention, is dependent on the transmembrane domain.

N-terminal deletions and His-tag fusions dramatically affect expression of cytochrome p450 2C2 in bacteria

The expression of mutants with deletions in the N-terminal signal-anchor sequence of cytochrome P450 2C2 and His-tag fusions was examined in Escherichia coli to determine the influence of N-terminal sequences on expression of the protein. Two mutants predicted to be translocated across the membrane inhibited bacterial growth. In other mutants, deletion of the N-terminal transmembrane domain (residues 2-20) reduced expression of functional P450 by about 75% and further deletion of the following linker sequence (residues 21-27) resulted in a modest further decrease. Expression of the mutant with residues 2-27 deleted contrasts with the lack of expression of functional protein if only the linker was deleted, which suggests that the linker sequence is critical for expression only if the protein is inserted into the membrane by the transmembrane domain. Fusion proteins of green fluorescent protein with full-length P450 2C2 and 2C2(Delta2-20) were predominantly membrane-associated in vivo as determined by fluorescence microscopy. Subcellular fractionation of bacteria expressing these proteins and extraction of the proteins from the membrane by high salt or alkaline buffer demonstrated that P450 2C2 was an integral membrane protein while 2C2(Delta2-20) was a peripheral membrane protein that associated with the membrane mainly by hydrophobic interactions. Residues 1-27 of P450 2C2 fused to green fluorescent protein resulted in a redistribution of fluorescence from cytosol to membrane, which, with the deletion studies, indicates that the P450 signal-anchor is both necessary and sufficient for normal membrane targeting and is the sole transmembrane domain of cytochrome P450 2C2 in bacteria. Addition of a His-tag at the N-terminus completely restored wild-type expression levels to the 2C2(Delta2-20) mutants in bacteria. In insect cells, functional 2C2(Delta2-20) was not expressed but an N-terminal His-tag also restored full expression. The increase in expression may be related to decreased association with the membrane mediated by the His-tag.

Diffusion in the endoplasmic reticulum of an aquaporin-2 mutant causing human nephrogenic diabetes insipidus

Mutations in the aquaporin-2 (AQP2) water channel cause the hereditary renal disease nephrogenic diabetes insipidus (NDI). The missense mutation AQP2-T126M causes human recessive NDI by retention at the endoplasmic reticulum (ER) of renal epithelial cells. To determine whether the ER retention of AQP2-T126M is due to relative immobilization in the ER, we measured by fluorescence recovery after photobleaching the intramembrane mobility of green fluorescent protein (GFP) chimeras containing human wild-type and mutant AQP2. In transfected LLC-PK1 renal epithelial cells, GFP-labeled AQP2-T126M was localized to the ER, and wild-type AQP2 to endosomes and the plasma membrane; both were localized to the ER after brefeldin A treatment. Photobleaching with image detection indicated that the GFP-AQP2 chimeras were freely mobile throughout the ER. Quantitative spot photobleaching revealed a diffusion-dependent irreversible process whose recovery depended on spot size and was abolished by paraformaldehyde fixation. In addition, a novel slow reversible fluorescence recovery (t(12) approximately 2 s) was characterized whose recovery was independent of spot size and not affected by fixation. AQP2 translational diffusion in the ER was not slowed by the T126M mutation; diffusion coefficients were (in cm(2)/s x 10(-)10) 2.6 +/- 0.5 (wild-type) and 3.0 +/- 0.4 (T126M). Much faster diffusion was found for a lipid probe (diOC(4)(3), 2.7 x 10(-)8 cm(2)/s) in the ER membrane and for unconjugated GFP in the aqueous ER lumen (6 x 10(-)8 cm(2)/s). ER diffusion of GFP-T126M was not significantly affected by up-regulation of molecular chaperones, cAMP activation, or actin filament disruption. ATP depletion by 2-deoxyglucose and azide resulted in comparable slowing/immobilization of wild-type and T126M AQP2. These results indicate that the ER retention of AQP2-T126M does not result from restricted or slowed mobility and suggest that the majority of AQP2-T126M is not aggregated or bound to slowly moving membrane proteins.

Subdomain-specific localization of CLIMP-63 (p63) in the endoplasmic reticulum is mediated by its luminal alpha-helical segment

The microtubule-binding integral 63 kD cytoskeleton-linking membrane protein (CLIMP-63; former name, p63) of the rough endoplasmic reticulum (ER) is excluded from the nuclear envelope. We studied the mechanism underlying this ER subdomain-specific localization by mutagenesis and structural analysis. Deleting the luminal but not cytosolic segment of CLIMP-63 abrogated subdomain-specific localization, as visualized by confocal microscopy in living cells and by immunoelectron microscopy using ultrathin cryosections. Photobleaching/recovery analysis revealed that the luminal segment determines restricted diffusion and immobility of the protein. The recombinant full-length luminal segment of CLIMP-63 formed alpha-helical 91-nm long rod-like structures as evident by circular dichroism spectroscopy and electron microscopy. In the analytical ultracentrifuge, the luminal segment sedimented at 25.7 S, indicating large complexes. The complexes most likely arose by electrostatic interactions of individual highly charged coiled coils. The findings indicate that the luminal segment of CLIMP-63 is necessary and sufficient for oligomerization into alpha-helical complexes that prevent nuclear envelope localization. Concentration of CLIMP-63 into patches may enhance microtubule binding on the cytosolic side and contribute to ER morphology by the formation of a protein scaffold in the lumen of the ER.

Diffusion in inhomogeneous media: theory and simulations applied to whole cell photobleach recovery

A continuum description for diffusion in a simple model for an inhomogeneous but isotropic media is derived and implemented numerically. The locally averaged density of diffusible marker is input from experiment to define the sample. Then a single additional parameter, the effective diffusion constant, permits the quantitative simulation of diffusive relaxation from any initial condition. Using this simulation, it is possible to model the recovery of a fluorescently tagged protein in the endoplasmic reticulum (ER) after photobleaching a substantial region of a live cell, and fit an effective diffusion constant which is a property both of the geometry of the ER and the marker. Such quantitative measurements permit inferences about the topology and internal organization of this organelle.

Endoplasmic reticulum retention determinants in the transmembrane and linker domains of cytochrome P450 2C1

The cytochrome P450 2C1 N-terminal signal anchor sequence mediates direct retention of the protein in the endoplasmic reticulum and consists of a hydrophobic transmembrane domain, residues 3-20, followed by a hydrophilic linker, residues 21-28. Fusions of the N-terminal 21 or 28 amino acids of P450 2C1 to green fluorescent protein resulted in endoplasmic reticulum localization of the chimera in transfected cells. Disruption of microtubules by nocodazole treatment resulted in redistribution into a punctate pattern for the 1-21, but not for the 1-28, chimera indicating that the linker was preventing transport from the endoplasmic reticulum but was not required for retrieval to the endoplasmic reticulum from the pre-Golgi compartment. In the 1-28 chimera, mutations of residues 21-23 (KQS) in the linker resulted in redistribution of the chimera after nocodazole treatment. Mutations in the transmembrane domain affected both direct retention in the endoplasmic reticulum and retrieval from the pre-Golgi compartment, and although structural requirements for each process are distinct, in both cases the arrangement of amino acids and distribution of hydrophobicity are critical. In contrast, the linker region exhibits a sequence-specific requirement for direct retention in the endoplasmic reticulum.

Molecular basis for the transport of cytochrome P450 2E1 to the plasma membrane

Endoplasmic reticulum-resident cytochrome P450 enzymes that face the cytosol are present on the plasma membrane of hepatocytes, but the molecular origin for their transport to this compartment has until now remained unknown. The molecular basis for the transport of rat ethanol-inducible cytochrome P450 2E1 (CYP2E1) to the plasma membrane was investigated by transfection of several different mutant cDNAs into mouse H2.35 hepatoma cells. Two NH(2)-terminal CYP2E1 mutants were constructed: N(++)2E1, which carried two positive charges in the NH(2) terminus, and 2C-2E1, in which the transmembrane domain of CYP2E1 was replaced with that of CYP2C1, which was previously described to cause retention of CYP2C1 in the endoplasmic reticulum, as well as CYP2E1 COOH-terminally tagged with the vesicular stomatitis virus G protein (VSV-G) epitope (2E1-VSV-G). Immunofluorescent microscopy and cell surface biotinylation experiments revealed that all CYP2E1 variants were present on the extracellular side of the plasma membrane. The VSV-G epitope on CYP2E1 was detected on the outside of the plasma membrane using VSV-G-specific antibodies, indicating that the large COOH-terminal part of CYP2E1 is indeed exposed on the outside of the plasma membrane. The relative levels of CYP2E1, 2C-2E1, and 2E1-VSV-G on the cell surface were found to be about 2% of total cellular enzyme, whereas twice this amount of N(++)2E1 was recovered at the cell surface. Protease protection experiments performed on microsomes isolated from cDNA transfected cells revealed that a small fraction of CYP2E1 and all variant proteins was found to be located in the lumen of the endoplasmic reticulum (type II orientation), whereas the majority of the proteins were in the expected cytosolic or type I orientation. It is concluded that the NH(2)-terminal transmembrane domain of CYP2E1 plays a critical role in directing the protein to the cell surface and that topological inversion of a small fraction of CYP2E1 in the endoplasmic reticulum directs the protein to the plasma membrane.

Colocalization of prostacyclin synthase with prostaglandin H synthase-1 (PGHS-1) but not phorbol ester-induced PGHS-2 in cultured endothelial cells

The subcellular colocalization of prostacyclin synthase (PGIS) with prostaglandin H synthase (PGHS) has not been delineated. To test the hypothesis that its colocalization with PGHS is crucial for prostacyclin synthesis, we determined subcellular locations of PGIS, PGHS-1, and PGHS-2 in bovine aortic endothelial cells by immunofluorescent confocal microscopy. PGIS and PGHS-1 were colocalized to nuclear envelope (NE) and endoplasmic reticulum (ER) in resting and adenovirus-infected bovine aortic endothelial cells. PGIS and PGHS-2 were also colocalized to ER in serum-treated or adenovirus-cyclooxygenase-2-infected cells. By contrast, PGIS was not colocalized with PGHS-2 in cells induced with phorbol 12-myristate 13-acetate where PGHS-2 was visualized primarily in vesicle-like structures. The lack of colocalization was accompanied by failed prostacyclin production. Resting ECV304 cells did not produce prostacyclin and had no detectable PGHS-1 and PGIS proteins. Confocal analysis showed abnormal colocalization of PGIS and PGHS-1 to a filamentous structure. Interestingly, the abundant PGIS and PGHS-1 expressed in adenovirus-infected ECV304 cells were colocalized to NE and ER, which synthesized a large quantity of prostacyclin. These findings underscore the importance of colocalization of PGHS and PGIS to ER and NE in prostacyclin synthesis.

Dynamics and retention of misfolded proteins in native ER membranes

When co-translationally inserted into endoplasmic reticulum (ER) membranes, newly synthesized proteins encounter the lumenal environment of the ER, which contains chaperone proteins that facilitate the folding reactions necessary for protein oligomerization, maturation and export from the ER. Here we show, using a temperature-sensitive variant of vesicular stomatitis virus G protein tagged with green fluorescent protein (VSVG-GFP), and fluorescence recovery after photobleaching (FRAP), the dynamics of association of folded and misfolded VSVG complexes with ER chaperones. We also investigate the potential mechanisms underlying protein retention in the ER. Misfolded VSVG-GFP complexes at 40 degrees C are highly mobile in ER membranes and do not reside in post-ER compartments, indicating that they are not retained in the ER by immobilization or retrieval mechanisms. These complexes are immobilized in ATP-depleted or tunicamycin-treated cells, in which VSVG-chaperone interactions are no longer dynamic. These results provide insight into the mechanisms of protein retention in the ER and the dynamics of protein-folding complexes in native ER membranes.