The NH2 terminus of retinal recoverin is acylated by a small family of fatty acids

Farnesylation and lipid unsaturation are critical for the membrane binding of the C-terminal segment of G-Protein Receptor Kinase 1

Many proteins are modified by the covalent addition of different types of lipids, such as myristoylation, palmitoylation and prenylation. Lipidation is expected to promote membrane association of proteins. Visual phototransduction involves many lipid-modified proteins. The G-Protein-coupled receptor of rod photoreceptors, rhodopsin, is inactivated by G-Protein-coupled Receptor Kinase 1 (GRK1). The C-terminus of GRK1 is farnesylated and its truncation has been shown to result in a very high decrease of its enzymatic activity, most likely because of the loss of its membrane localization. Little information is available on the membrane binding of GRK1 as well as of most prenylated proteins. Measurements of the membrane binding of the non-farnesylated and farnesylated C-terminal segment of GRK1 were thus performed using lipids typical of those found in rod outer segment disk membranes. Their random coil secondary structure was determined using circular dichroism and infrared spectroscopy. The non-farnesylated C-terminal segment of GRK1 has no surface activity. In contrast, the farnesylated C-terminal segment of GRK1 shows a particularly strong binding to lipid monolayers bearing at least one unsaturated fatty acyl chain. No binding is observed in the presence of monolayers of saturated phospholipids, in agreement with the low affinity of farnesylated Ras proteins for lipids in the liquid-ordered state. Altogether, these data demonstrate that the farnesyl group of the C-terminal segment of GRK1 is mandatory for its membrane binding, which is favored by particular lipids or lipid mixtures. This information will also be useful for the understanding of the membrane binding of other prenylated proteins.

Protein Lipidation Types: Current Strategies for Enrichment and Characterization

Post-translational modifications regulate diverse activities of a colossal number of proteins. For example, various types of lipids can be covalently linked to proteins enzymatically or non-enzymatically. Protein lipidation is perhaps not as extensively studied as protein phosphorylation, ubiquitination, or glycosylation although it is no less significant than these modifications. Evidence suggests that proteins can be attached by at least seven types of lipids, including fatty acids, lipoic acids, isoprenoids, sterols, phospholipids, glycosylphosphatidylinositol anchors, and lipid-derived electrophiles. In this review, we summarize types of protein lipidation and methods used for their detection, with an emphasis on the conjugation of proteins with polyunsaturated fatty acids (PUFAs). We discuss possible reasons for the scarcity of reports on PUFA-modified proteins, limitations in current methodology, and potential approaches in detecting PUFA modifications.

Functional compartmentalization of photoreceptor neurons

Retinal photoreceptors are neurons that convert dynamically changing patterns of light into electrical signals that are processed by retinal interneurons and ultimately transmitted to vision centers in the brain. They represent the essential first step in seeing without which the remainder of the visual system is rendered moot. To support this role, the major functions of photoreceptors are segregated into three main specialized compartments-the outer segment, the inner segment, and the pre-synaptic terminal. This compartmentalization is crucial for photoreceptor function-disruption leads to devastating blinding diseases for which therapies remain elusive. In this review, we examine the current understanding of the molecular and physical mechanisms underlying photoreceptor functional compartmentalization and highlight areas where significant knowledge gaps remain.

Compartmentalization of Photoreceptor Sensory Cilia

Functional compartmentalization of cells is a universal strategy for segregating processes that require specific components, undergo regulation by modulating concentrations of those components, or that would be detrimental to other processes. Primary cilia are hair-like organelles that project from the apical plasma membranes of epithelial cells where they serve as exclusive compartments for sensing physical and chemical signals in the environment. As such, molecules involved in signal transduction are enriched within cilia and regulating their ciliary concentrations allows adaptation to the environmental stimuli. The highly efficient organization of primary cilia has been co-opted by major sensory neurons, olfactory cells and the photoreceptor neurons that underlie vision. The mechanisms underlying compartmentalization of cilia are an area of intense current research. Recent findings have revealed similarities and differences in molecular mechanisms of ciliary protein enrichment and its regulation among primary cilia and sensory cilia. Here we discuss the physiological demands on photoreceptors that have driven their evolution into neurons that rely on a highly specialized cilium for signaling changes in light intensity. We explore what is known and what is not known about how that specialization appears to have driven unique mechanisms for photoreceptor protein and membrane compartmentalization.

Regulation of retinal membrane guanylyl cyclase (RetGC) by negative calcium feedback and RD3 protein

This article presents a brief overview of the main biochemical and cellular processes involved in regulation of cyclic GMP production in photoreceptors. The main focus is on how the fluctuations of free calcium concentrations in photoreceptors between light and dark regulate the activity of retinal membrane guanylyl cyclase (RetGC) via calcium sensor proteins. The emphasis of the review is on the structure of RetGC and guanylyl cyclase activating proteins (GCAPs) in relation to their functional role in photoreceptors and congenital diseases of photoreceptors. In addition to that, the structure and function of retinal degeneration-3 protein (RD3), which regulates RetGC in a calcium-independent manner, is discussed in detail in connections with its role in photoreceptor biology and inherited retinal blindness.

Bringing the Ca2+ sensitivity of myristoylated recoverin into the physiological range

The prototypical Ca²⁺-sensor protein recoverin (Rec) is thought to regulate the activity of rhodopsin kinase (GRK1) in photoreceptors by switching from a relaxed (R) disc membrane-bound conformation in the dark to a more compact, cytosol-diffusing tense (T) conformation upon cell illumination. However, the apparent affinity for Ca²⁺ of its physiologically relevant form (myristoylated recoverin) is almost two orders of magnitude too low to support this mechanism in vivo. In this work, we compared the individual and synergistic roles of the myristic moiety, the GRK1 target and the disc membrane in modulating the calcium sensitivity of Rec. We show that the sole presence of the target or the disc membrane alone are not sufficient to achieve a physiological response to changes in intracellular [Ca²⁺]. Instead, the simultaneous presence of GRK1 and membrane allows the T to R transition to occur in a physiological range of [Ca²⁺] with high cooperativity via a conformational selection mechanism that drives the structural transitions of Rec in the presence of multiple ligands. Our conclusions may apply to other sensory transduction systems involving protein complexes and biological membranes.

Interaction of G protein-coupled receptor kinases and recoverin isoforms is determined by localization in zebrafish photoreceptors

The zebrafish retina expresses four recoverin genes (rcv1a, rcv1b, rcv2a and rcv2b) and four opsin kinase genes (grk1a, grk1b, grk7a and grk7b) coding for recoverin and G protein-coupled receptor kinase (opsin kinase) paralogs, respectively. Both protein groups are suggested to form regulatory complexes in rod and cone outer segments, but at present, we lack information about co-localization of recoverin and opsin kinases in zebrafish retinae and which protein-protein interacting pairs form. We analyzed the distribution and co-localization of recoverin and opsin kinase expression in the zebrafish retina. For this purpose, we used custom-tailored monospecific antibodies revealing that the amount of recoverin paralogs in a zebrafish retina can differ by more than one order of magnitude with the highest amount for recoverin 1a and 2b. Further, immunohistochemical labelling showed presence of recoverin 1a in all rod cell compartments, but it only co-localized with opsin kinase 1a in rod outer segments. In contrast, recoverin 2b was only detected in double cones and co-localized with opsin kinases 1b, 7a and 7b. Further, we investigated the interaction between recoverin and opsin kinase variants by surface plasmon resonance spectroscopy indicating interaction of recoverin 1a and recoverin 2b with all opsin kinases. However, binding kinetics for recoverin 1a differed from those observed with recoverin 2b that showed slower association and dissociation processes. Our results indicate diverse recoverin and opsin kinase properties due to differential expression and interaction profiles.

Effects of Membrane and Biological Target on the Structural and Allosteric Properties of Recoverin: A Computational Approach

Recoverin (Rec) is a prototypical calcium sensor protein primarily expressed in the vertebrate retina. The binding of two Ca²⁺ ions to the functional EF-hand motifs induces the extrusion of a myristoyl group that increases the affinity of Rec for the membrane and leads to the formation of a complex with rhodopsin kinase (GRK1). Here, unbiased all-atom molecular dynamics simulations were performed to monitor the spontaneous insertion of the myristoyl group into a model multicomponent biological membrane for both isolated Rec and for its complex with a peptide from the GRK1 target. It was found that the functional membrane anchoring of the myristoyl group is triggered by persistent electrostatic protein-membrane interactions. In particular, salt bridges between Arg43, Arg46 and polar heads of phosphatidylserine lipids are necessary to enhance the myristoyl hydrophobic packing in the Rec-GRK1 assembly. The long-distance communication between Ca²⁺-binding EF-hands and residues at the interface with GRK1 is significantly influenced by the presence of the membrane, which leads to dramatic changes in the connectivity of amino acids mediating the highest number of persistent interactions (hubs). In conclusion, specific membrane composition and allosteric interactions are both necessary for the correct assembly and dynamics of functional Rec-GRK1 complex.

Experimental Insight into the Structural and Functional Roles of the 'Black' and 'Gray' Clusters in Recoverin, a Calcium Binding Protein with Four EF-Hand Motifs

Recently, we have found that calcium binding proteins of the EF-hand superfamily (i.e., a large family of proteins containing helix-loop-helix calcium binding motif or EF-hand) contain two types of conserved clusters called cluster I ('black' cluster) and cluster II ('grey' cluster), which provide a supporting scaffold for the Ca2+ binding loops and contribute to the hydrophobic core of the EF-hand domains. Cluster I is more conservative and mostly incorporates aromatic amino acids, whereas cluster II includes a mix of aromatic, hydrophobic, and polar amino acids of different sizes. Recoverin is EF-hand Ca2+-binding protein containing two 'black' clusters comprised of F35, F83, Y86 (N-terminal domain) and F106, E169, F172 (C-terminal domain) as well as two 'gray' clusters comprised of F70, Q46, F49 (N-terminal domain) and W156, K119, V122 (C-terminal domain). To understand a role of these residues in structure and function of human recoverin, we sequentially substituted them for alanine and studied the resulting mutants by a set of biophysical methods. Under metal-free conditions, the 'black' clusters mutants (except for F35A and E169A) were characterized by an increase in the α-helical content, whereas the 'gray' cluster mutants (except for K119A) exhibited the opposite behavior. By contrast, in Ca2+-loaded mutants the α-helical content was always elevated. In the absence of calcium, the substitutions only slightly affected multimerization of recoverin regardless of their localization (except for K119A). Meanwhile, in the presence of calcium mutations in N-terminal domain of the protein significantly suppressed this process, indicating that surface properties of Ca2+-bound recoverin are highly affected by N-terminal cluster residues. The substitutions in C-terminal clusters generally reduced thermal stability of recoverin with F172A ('black' cluster) as well as W156A and K119A ('gray' cluster) being the most efficacious in this respect. In contrast, the mutations in the N-terminal clusters caused less pronounced differently directed changes in thermal stability of the protein. The substitutions of F172, W156, and K119 in C-terminal domain of recoverin together with substitution of Q46 in its N-terminal domain provoked significant but diverse changes in free energy associated with Ca2+ binding to the protein: the mutant K119A demonstrated significantly improved calcium binding, whereas F172A and W156A showed decrease in the calcium affinity and Q46A exhibited no ion coordination in one of the Ca2+-binding sites. The most of the N-terminal clusters mutations suppressed membrane binding of recoverin and its inhibitory activity towards rhodopsin kinase (GRK1). Surprisingly, the mutant W156A aberrantly activated rhodopsin phosphorylation regardless of the presence of calcium. Taken together, these data confirm the scaffolding function of several cluster-forming residues and point to their critical role in supporting physiological activity of recoverin.

ACBD6 protein controls acyl chain availability and specificity of the N-myristoylation modification of proteins

Members of the human acyl-CoA binding domain-containing (ACBD) family regulate processes as diverse as viral replication, stem-cell self-renewal, organelle organization, and protein acylation. These functions are defined by nonconserved motifs present downstream of the ACBD. The human ankyrin-repeat-containing ACBD6 protein supports the reaction catalyzed by the human and Plasmodium N-myristoyltransferase (NMT) enzymes. Likewise, the newly identified Plasmodium ACBD6 homologue regulates the activity of the NMT enzymes. The relatively low abundance of myristoyl-CoA in the cell limits myristoylation. Binding of myristoyl-CoA to NMT is competed by more abundant acyl-CoA species such as palmitoyl-CoA. ACBD6 also protects the Plasmodium NMT enzyme from lauryl-CoA and forces the utilization of the myristoyl-CoA substrate. The phosphorylation of two serine residues of the acyl-CoA binding domain of human ACBD6 improves ligand binding capacity, prevents competition by unbound acyl-CoAs, and further enhances the activity of NMT. Thus, ACBD6 proteins promote N-myristoylation in mammalian cells and in one of their intracellular parasites under unfavorable substrate-limiting conditions.

Binding of a Myristoylated Protein to the Lipid Membrane Influenced by Interactions with the Polar Head Group Region

Many cytoplasmic proteins contain a hydrophobic acyl chain, which facilitates protein binding to cell membranes. Hydrophobic interactions between the exposed acyl chain of the protein and hydrocarbon chains of lipids in the cell membrane are the driving force for this specific lipid-protein interaction. Recent studies point out that in addition to hydrophobic interactions the charge-charge and charge-dipole interactions between the polar head groups and basic amino acids contribute significantly to the binding process. Recoverin possesses a myristoyl chain at the N-terminus. In the presence of Ca²⁺ ions, the protein undergoes structural rearrangements, leading to the extrusion of the myristoyl chain, facilitating the protein binding to the membrane. In this work, we investigate the impact of interactions between the polar head group region of lipid molecules and recoverin which binds to the model membrane. The interaction with a planar lipid bilayer composed of phosphatidylcholine and cholesterol with myristoylated and nonmyristoylated recoverin is studied by in situ polarization modulation infrared reflection absorption spectroscopy. The binding of recoverin to the lipid bilayer depends on the transmembrane potential, indicating that the orientation of the permanent surface dipole in the supramolecular assembly of the lipid membrane influences the protein attachment to the membrane surface. Analysis of the amide I' mode indicates that the orientation of recoverin bound to the lipid bilayer is independent of the presence of myristoyl chain in the protein and of the folding of the protein into the tense or relaxed state. In contrast, it changes as a function of the membrane potential. At positive transmembrane potentials, the α-helical fragments of recoverin are oriented predominantly parallel to the bilayer surface. This orientation facilitates the insertion of the acyl chain of the protein into the hydrophobic region of the bilayer. At negative transmembrane potentials, the α-helical fragments of recoverin change their orientation with respect to the membrane surface, which is followed by the removal of the myristoyl chain from the membrane.

Dimerization of Neuronal Calcium Sensor Proteins

Neuronal calcium sensor (NCS) proteins are EF-hand containing Ca²⁺ binding proteins that regulate sensory signal transduction. Many NCS proteins (recoverin, GCAPs, neurocalcin and visinin-like protein 1 (VILIP1)) form functional dimers under physiological conditions. The dimeric NCS proteins have similar amino acid sequences (50% homology) but each bind to and regulate very different physiological targets. Retinal recoverin binds to rhodopsin kinase and promotes Ca²⁺-dependent desensitization of light-excited rhodopsin during visual phototransduction. The guanylyl cyclase activating proteins (GCAP1–5) each bind and activate retinal guanylyl cyclases (RetGCs) in light-adapted photoreceptors. VILIP1 binds to membrane targets that modulate neuronal secretion. Here, I review atomic-level structures of dimeric forms of recoverin, GCAPs and VILIP1. The distinct dimeric structures in each case suggest that NCS dimerization may play a role in modulating specific target recognition. The dimerization of recoverin and VILIP1 is Ca²⁺-dependent and enhances their membrane-targeting Ca²⁺-myristoyl switch function. The dimerization of GCAP1 and GCAP2 facilitate their binding to dimeric RetGCs and may allosterically control the Ca²⁺-dependent activation of RetGCs.

Protein Lipidation: Occurrence, Mechanisms, Biological Functions, and Enabling Technologies

Protein lipidation, including cysteine prenylation, N-terminal glycine myristoylation, cysteine palmitoylation, and serine and lysine fatty acylation, occurs in many proteins in eukaryotic cells and regulates numerous biological pathways, such as membrane trafficking, protein secretion, signal transduction, and apoptosis. We provide a comprehensive review of protein lipidation, including descriptions of proteins known to be modified and the functions of the modifications, the enzymes that control them, and the tools and technologies developed to study them. We also highlight key questions about protein lipidation that remain to be answered, the challenges associated with answering such questions, and possible solutions to overcome these challenges.

Structural Characterization of Ferrous Ion Binding to Retinal Guanylate Cyclase Activator Protein 5 from Zebrafish Photoreceptors

Sensory guanylate cyclases (zGCs) in zebrafish photoreceptors are regulated by a family of guanylate cyclase activator proteins (called GCAP1-7). GCAP5 contains two nonconserved cysteine residues (Cys15 and Cys17) that could in principle bind to biologically active transition state metal ions (Zn²⁺ and Fe²⁺). Here, we present nuclear magnetic resonance (NMR) and isothermal titration calorimetry (ITC) binding analyses that demonstrate the binding of one Fe²⁺ ion to two GCAP5 molecules (in a 1:2 complex) with a dissociation constant in the nanomolar range. At least one other Fe²⁺ binds to GCAP5 with micromolar affinity that likely represents electrostatic Fe²⁺ binding to the EF-hand loops. The GCAP5 double mutant (C15A/C17A) lacks nanomolar binding to Fe²⁺, suggesting that Fe²⁺ at this site is ligated directly by thiolate groups of Cys15 and Cys17. Size exclusion chromatography analysis indicates that GCAP5 forms a dimer in the Fe²⁺-free and Fe²⁺-bound states. NMR structural analysis and molecular docking studies suggest that a single Fe²⁺ ion is chelated by thiol side chains from Cys15 and Cys17 in the GCAP5 dimer, forming an [Fe(SCys)₄] complex like that observed previously in two-iron superoxide reductases. Binding of Fe²⁺ to GCAP5 weakens its ability to activate photoreceptor human GC-E by decreasing GC activity >10-fold. Our results indicate a strong Fe²⁺-induced inhibition of GC by GCAP5 and suggest that GCAP5 may serve as a redox sensor in visual phototransduction.

Association of NMT2 with the acyl-CoA carrier ACBD6 protects the N-myristoyltransferase reaction from palmitoyl-CoA

The covalent attachment of a 14-carbon aliphatic tail on a glycine residue of nascent translated peptide chains is catalyzed in human cells by two N-myristoyltransferase (NMT) enzymes using the rare myristoyl-CoA (C(14)-CoA) molecule as fatty acid donor. Although, NMT enzymes can only transfer a myristate group, they lack specificity for C(14)-CoA and can also bind the far more abundant palmitoyl-CoA (C(16)-CoA) molecule. We determined that the acyl-CoA binding protein, acyl-CoA binding domain (ACBD)6, stimulated the NMT reaction of NMT2. This stimulatory effect required interaction between ACBD6 and NMT2, and was enhanced by binding of ACBD6 to its ligand, C(18:2)-CoA. ACBD6 also interacted with the second human NMT enzyme, NMT1. The presence of ACBD6 prevented competition of the NMT reaction by C(16)-CoA. Mutants of ACBD6 that were either deficient in ligand binding to the N-terminal ACBD or unable to interact with NMT2 did not stimulate activity of NMT2, nor could they protect the enzyme from utilizing the competitor C(16)-CoA. These results indicate that ACBD6 can locally sequester C(16)-CoA and prevent its access to the enzyme binding site via interaction with NMT2. Thus, the ligand binding properties of the NMT/ACBD6 complex can explain how the NMT reaction can proceed in the presence of the very abundant competitive substrate, C(16)-CoA.

Protein and Signaling Networks in Vertebrate Photoreceptor Cells

Vertebrate photoreceptor cells are exquisite light detectors operating under very dim and bright illumination. The photoexcitation and adaptation machinery in photoreceptor cells consists of protein complexes that can form highly ordered supramolecular structures and control the homeostasis and mutual dependence of the secondary messengers cyclic guanosine monophosphate (cGMP) and Ca²⁺. The visual pigment in rod photoreceptors, the G protein-coupled receptor rhodopsin is organized in tracks of dimers thereby providing a signaling platform for the dynamic scaffolding of the G protein transducin. Illuminated rhodopsin is turned off by phosphorylation catalyzed by rhodopsin kinase (GRK1) under control of Ca²⁺-recoverin. The GRK1 protein complex partly assembles in lipid raft structures, where shutting off rhodopsin seems to be more effective. Re-synthesis of cGMP is another crucial step in the recovery of the photoresponse after illumination. It is catalyzed by membrane bound sensory guanylate cyclases (GCs) and is regulated by specific neuronal Ca²⁺-sensor proteins called guanylate cyclase-activating proteins (GCAPs). At least one GC (ROS-GC1) was shown to be part of a multiprotein complex having strong interactions with the cytoskeleton and being controlled in a multimodal Ca²⁺-dependent fashion. The final target of the cGMP signaling cascade is a cyclic nucleotide-gated (CNG) channel that is a hetero-oligomeric protein located in the plasma membrane and interacting with accessory proteins in highly organized microdomains. We summarize results and interpretations of findings related to the inhomogeneous organization of signaling units in photoreceptor outer segments.

Structural diversity of neuronal calcium sensor proteins and insights for activation of retinal guanylyl cyclase by GCAP1

Neuronal calcium sensor (NCS) proteins, a sub-branch of the calmodulin superfamily, are expressed in the brain and retina where they transduce calcium signals and are genetically linked to degenerative diseases. The amino acid sequences of NCS proteins are highly conserved but their physiological functions are quite different. Retinal recoverin controls Ca²⁺-dependent inactivation of light-excited rhodopsin during phototransduction, guanylyl cyclase activating proteins 1 and 2 (GCAP1 and GCAP2) promote Ca²⁺-dependent activation of retinal guanylyl cyclases, and neuronal frequenin (NCS-1) modulates synaptic activity and neuronal secretion. Here we review the molecular structures of myristoylated forms of NCS-1, recoverin, and GCAP1 that all look very different, suggesting that the attached myristoyl group helps to refold these highly homologous proteins into different three-dimensional folds. Ca²⁺-binding to both recoverin and NCS-1 cause large protein conformational changes that ejects the covalently attached myristoyl group into the solvent exterior and promotes membrane targeting (Ca²⁺-myristoyl switch). The GCAP proteins undergo much smaller Ca²⁺-induced conformational changes and do not possess a Ca²⁺-myristoyl switch. Recent structures of GCAP1 in both its activator and Ca²⁺-bound inhibitory states will be discussed to understand structural determinants that control their Ca²⁺-dependent activation of retinal guanylyl cyclases.

A highly conserved cysteine of neuronal calcium-sensing proteins controls cooperative binding of Ca2+ to recoverin

Recoverin, a 23-kDa Ca(2+)-binding protein of the neuronal calcium sensing (NCS) family, inhibits rhodopsin kinase, a Ser/Thr kinase responsible for termination of photoactivated rhodopsin in rod photoreceptor cells. Recoverin has two functional EF hands and a myristoylated N terminus. The myristoyl chain imparts cooperativity to the Ca(2+)-binding sites through an allosteric mechanism involving a conformational equilibrium between R and T states of the protein. Ca(2+) binds preferentially to the R state; the myristoyl chain binds preferentially to the T state. In the absence of myristoylation, the R state predominates, and consequently, binding of Ca(2+) to the non-myristoylated protein is not cooperative. We show here that a mutation, C39A, of a highly conserved Cys residue among NCS proteins, increases the apparent cooperativity for binding of Ca(2+) to non-myristoylated recoverin. The binding data can be explained by an effect on the T/R equilibrium to favor the T state without affecting the intrinsic binding constants for the two Ca(2+) sites.

Protein sorting, targeting and trafficking in photoreceptor cells

Vision is the most fundamental of our senses initiated when photons are absorbed by the rod and cone photoreceptor neurons of the retina. At the distal end of each photoreceptor resides a light-sensing organelle, called the outer segment, which is a modified primary cilium highly enriched with proteins involved in visual signal transduction. At the proximal end, each photoreceptor has a synaptic terminal, which connects this cell to the downstream neurons for further processing of the visual information. Understanding the mechanisms involved in creating and maintaining functional compartmentalization of photoreceptor cells remains among the most fascinating topics in ocular cell biology. This review will discuss how photoreceptor compartmentalization is supported by protein sorting, targeting and trafficking, with an emphasis on the best-studied cases of outer segment-resident proteins.

Molecular structure and target recognition of neuronal calcium sensor proteins

BACKGROUND

Neuronal calcium sensor (NCS) proteins, a sub-branch of the calmodulin superfamily, are expressed in the brain and retina where they transduce calcium signals and are genetically linked to degenerative diseases. The amino acid sequences of NCS proteins are highly conserved but their physiological functions are quite distinct. Retinal recoverin and guanylate cyclase activating proteins (GCAPs) both serve as calcium sensors in retinal rod cells, neuronal frequenin (NCS1) modulate synaptic activity and neuronal secretion, K+ channel interacting proteins (KChIPs) regulate ion channels to control neuronal excitability, and DREAM (KChIP3) is a transcriptional repressor that regulates neuronal gene expression.

SCOPE OF REVIEW

Here we review the molecular structures of myristoylated forms of NCS1, recoverin, and GCAP1 that all look very different, suggesting that the sequestered myristoyl group helps to refold these highly homologous proteins into very different structures. The molecular structure of NCS target complexes have been solved for recoverin bound to rhodopsin kinase, NCS-1 bound to phosphatidylinositol 4-kinase, and KChIP1 bound to A-type K+ channels.

MAJOR CONCLUSIONS

We propose the idea that N-terminal myristoylation is critical for shaping each NCS family member into a unique structure, which upon Ca2+-induced extrusion of the myristoyl group exposes a unique set of previously masked residues, thereby exposing a distinctive ensemble of hydrophobic residues to associate specifically with a particular physiological target. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

Operation profile of zebrafish guanylate cyclase-activating protein 3

The expression pattern and property profile of the neuronal Ca(2+) sensor guanylate cyclase-activating protein 3 (zGCAP3) was studied by immunochemical approaches, biophysical methods and enzymatic assays. Using affinity purified antibodies immunoreactivity towards zGCAP3 was weakly detected in the outer and strongly in the inner segments of cone cells as well as in the outer plexiform layer, to a lesser degree also in the inner plexiform and ganglion cell layer of the zebrafish retina. This cellular distribution was independent of a dark/light cycle. Some neuronal Ca(2+) sensors are acylated (mainly myristoylated) at the amino-terminus. Probing larval and adult stages of the developing zebrafish retina indicated that zGCAP3 was first expressed in a non-myristoylated form, but was finally present in the adult retina as a myristoylated protein. While zGCAP3 did not undergo a classical Ca(2+) -myristoyl switch as investigated by surface plasmon resonance spectroscopy, myristoylation had two main other consequences: it enhanced the Ca(2+) -sensitivity of the Ca(2+) -induced conformational change and it stabilized the protein conformation. Differences between myristoylated and non-myristoylated zGCAP3 were also observed in modulating the kinetic and catalytic parameters of the GCAP-target, a membrane bound guanylate cyclase. Thus, the stabilizing effect of the myristoyl group is apparently less important in the larval than in the adult fish.

Calcium-myristoyl Tug is a new mechanism for intramolecular tuning of calcium sensitivity and target enzyme interaction for guanylyl cyclase-activating protein 1: dynamic connection between N-fatty acyl group and EF-hand controls calcium sensitivity

Guanylyl cyclase-activating protein 1 (GCAP1), a myristoylated Ca(2+) sensor in vision, regulates retinal guanylyl cyclase (RetGC). We show that protein-myristoyl group interactions control Ca(2+) sensitivity, apparent affinity for RetGC, and maximal level of cyclase activation. Mutating residues near the myristoyl moiety affected the affinity of Ca(2+) binding to EF-hand 4. Inserting Phe residues in the cavity around the myristoyl group increased both the affinity of GCAP1 for RetGC and maximal activation of the cyclase. NMR spectra show that the myristoyl group in the L80F/L176F/V180F mutant remained sequestered inside GCAP1 in both Ca(2+)-bound and Mg(2+)-bound states. This mutant displayed much higher affinity for the cyclase but reduced Ca(2+) sensitivity of the cyclase regulation. The L176F substitution improved affinity of myristoylated and non-acylated GCAP1 for the cyclase but simultaneously reduced the affinity of Ca(2+) binding to EF-hand 4 and Ca(2+) sensitivity of the cyclase regulation by acylated GCAP1. The replacement of amino acids near both ends of the myristoyl moiety (Leu(80) and Val(180)) minimally affected regulatory properties of GCAP1. N-Lauryl- and N-myristoyl-GCAP1 activated RetGC in a similar fashion. Thus, protein interactions with the central region of the fatty acyl chain optimize GCAP1 binding to RetGC and maximize activation of the cyclase. We propose a dynamic connection (or "tug") between the fatty acyl group and EF-hand 4 via the C-terminal helix that attenuates the efficiency of RetGC activation in exchange for optimal Ca(2+) sensitivity.

Amino acid sequences of two immune-dominant epitopes of recoverin are involved in Ca2+/recoverin-dependent inhibition of phosphorylation of rhodopsin

Antibodies AB(60-72) and AB(80-92) against two immune-dominant epitopes of photoreceptor Ca(2+)-binding protein recoverin, 60-DPKAYAQHVFRSF-72 and 80-LDFKEYVIALHMT-92, which can be exposed in a Ca(2+)-dependent manner, were obtained. The presence of AB(60-72) or AB(80-92) results in a slight increase in Ca(2+)-affinity of recoverin and does not affect significantly a Ca(2+)-myristoyl switch mechanism of the protein. However in the presence of AB(60-72) or AB(80-92) recoverin loses its ability to interact with rhodopsin kinase and consequently to perform a function of Ca(2+)-sensitive inhibitor of rhodopsin phosphorylation in photoreceptor cells.

The myristoylation of guanylate cyclase-activating protein-2 causes an increase in thermodynamic stability in the presence but not in the absence of Ca²⁺

Guanylate cyclase activating protein-2 (GCAP-2) is a Ca²⁺-binding protein of the neuronal calcium sensor (NCS) family. Ca²⁺-free GCAP-2 activates the retinal rod outer segment guanylate cyclases ROS-GC1 and 2. Native GCAP-2 is N-terminally myristoylated. Detailed structural information on the Ca²⁺-dependent conformational switch of GCAP-2 is missing so far, as no atomic resolution structures of the Ca²⁺-free state have been determined. The role of the myristoyl moiety remains poorly understood. Available functional data is incompatible with a Ca²⁺-myristoyl switch as observed in the prototype NCS protein, recoverin. For the homologous GCAP-1, a Ca²⁺-independent sequestration of the myristoyl moiety inside the proteins structure has been proposed. In this article, we compare the thermodynamic stabilities of myristoylated and non-myristoylated GCAP-2 in their Ca²⁺-bound and Ca²⁺-free forms, respectively, to gain information on the nature of the Ca²⁺-dependent conformational switch of the protein and shed some light on the role of its myristoyl group. In the absence of Ca²⁺, the stability of the myristoylated and non-myristoylated forms was indistinguishable. Ca²⁺ exerted a stabilizing effect on both forms of the protein, which was significantly stronger for myr GCAP-2. The stability data were corroborated by dye binding experiments performed to probe the solvent-accessible hydrophobic surface of the protein. Our results strongly suggest that the myristoyl moiety is permanently solvent-exposed in Ca²⁺-free GCAP-2, whereas it interacts with a hydrophobic part of the protein's structure in the Ca²⁺-bound state.

Heterogeneous N-terminal acylation of retinal proteins results from the retina's unusual lipid metabolism

Protein N-myristoylation occurs by a covalent attachment of a C14:0 fatty acid to the N-terminal Gly residue. This reaction is catalyzed by a N-myristoyltransferase that uses myristoyl-coenzyme A as substrate. But proteins in the retina also undergo heterogeneous N-acylation with C14:2, C14:1, and C12:0 fatty acids. The basis and the role of this retina-specific phenomenon are poorly understood. We studied guanylate cyclase-activating protein 1 (GCAP1) as an example of retina-specific heterogeneously N-acylated protein. The types and the abundance of fatty acids bound to bovine retinal GCAP1 were C14:2, 37.0%; C14:0, 32.4%; C14:1, 22.3%; and C12:0, 8.3% as quantified by liquid chromatography coupled mass spectrometry. We also devised a method for N-acylating proteins in vitro and used it to modify GCAP1 with acyl moieties of different lengths. Analysis of these GCAPs both confirmed that N-terminal acylation of GCAP1 is critical for its high activity and proper Ca(2+)-dependent response and revealed comparable functionality for GCAP1 with acyl moieties of various lengths. We also tested the hypothesis that retinal heterogeneous N-acylation results from retinal enrichment of unusual N-myristoyltransferase substrates. Thus, acyl-coenzyme A esters were purified from both bovine retina and brain and analyzed by liquid chromatography coupled mass spectrometry. Substantial differences in acyl-coenzyme A profiles between the retina and brain were detected. Importantly, the ratios of uncommon N-acylation substrates--C14:2- and C14:1-coenyzme A to C14:0-coenzyme A--were higher in the retina than in the brain. Thus, our results suggest that heterogeneous N-acylation, responsible for expansion of retinal proteome, reflects the unique character of retinal lipid metabolism. Additionally, we propose a new hypothesis explaining the physiological relevance of elevated retinal ratios of C14:2- and C14:1-coenzyme A to C14:0-coenzyme A.

Free fatty acids profile of the fetal brain and the plasma, liver, brain and kidneys of pregnant rats treated with sodium arsenite at mid-organogenesis

Free fatty acids (FFAs) are known to be markers of cellular membrane degradation through lipid peroxidation and are substrates for the production of reactive oxygen species (ROS). Oxidative stress, due to overproduction of ROS, may facilitate cellular insult by various toxicants. The ability of the rat conceptus to respond to toxic stress may be critical for normal development. In this study, the effects of the environmental toxicant sodium arsenite (NaAsO₂) on FFAs were investigated after administering a single oral dose, in water and in a lipid medium, to pregnant rats on gestational day (GD) 10, a time point at mid-organogenesis. NaAsO₂ was administered in deionized water (AsH₂O) or in half and half dairy cream (AsHH) at a dose of 41 mg sodium arsenite (NaAsO₂)/kg body weight. Control animals were treated with either dairy cream (HH) or deionized water (H₂O). The animals were sacrificed on GD 20. The fetal brain and the maternal liver, brain, plasma and kidneys were harvested. The FFAs were extracted and analyzed by gas chromatography. In the liver, there was an increase of myristic acid (1200%), myristoleic acid (174%), palmitic acid (47%), elaidic acid (456%), oleic acid (165%) and docosahexaenoic acid (224%) in the AsH₂O group as compared to the AsHH group. Oleic acid and arachidonic acid were increased by 192% and 900%, respectively, in the AsH₂O group as compared to the H₂O group, and myristic acid was decreased by 90% in the AsHH group as compared to the HH group. In the maternal brain, myristoleic acid was decreased by 91% in the AsH₂O group as compared to the H₂O group, and DHA increased by 148% in the AsHH group as compared to the HH group. In the fetal brain, myristic and stearic acids were decreased by 87% and 89%, respectively, in the AsH₂O group as compared to the AsHH group. Myristic, stearic and arachidonic acids were increased by 411%, 265%, and 144%, respectively, in the AsHH group as compared to the HH group. There was no effect on the fatty acids concentrations in the kidney or plasma as compared to controls. This study shows that NaAsO₂ produced a differential effect on the fatty acid profiles in rats. Further investigation is needed to elucidate the role of fatty acids in differential signaling and regulation by either the palmitoylation or myristoylation process of cellular functions in these target organs.

Determination of the contribution of the myristoyl group and hydrophobic amino acids of recoverin on its dynamics of binding to lipid monolayers

It has been postulated that myristoylation of peripheral proteins would facilitate their binding to membranes. However, the exact involvement of this lipid modification in membrane binding is still a matter of debate. Proteins containing a Ca(2+)-myristoyl switch where the extrusion of their myristoyl group is dependent on calcium binding is best illustrated by the Ca(2+)-binding recoverin, which is present in retinal rod cells. The parameters responsible for the modulation of the membrane binding of recoverin are still largely unknown. This study was thus performed to determine the involvement of different parameters on recoverin membrane binding. We have used surface pressure measurements and PM-IRRAS spectroscopy to monitor the adsorption of myristoylated and nonmyristoylated recoverin onto phospholipid monolayers in the presence and absence of calcium. The adsorption curves have shown that the myristoyl group and hydrophobic residues of myristoylated recoverin strongly accelerate membrane binding in the presence of calcium. In the case of nonmyristoylated recoverin in the presence of calcium, hydrophobic residues alone are responsible for its much faster monolayer binding than myristoylated and nonmyristoylated recoverin in the absence of calcium. The infrared spectra revealed that myristoylated and nonmyristoylated recoverin behave very different upon adsorption onto phospholipid monolayers. Indeed, PM-IRRAS spectra indicated that the myristoyl group allows a proper orientation and organization as well as faster and stronger binding of myristoylated recoverin to lipid monolayers compared to nonmyristoylated recoverin. Simulations of the spectra have allowed us to postulate that nonmyristoylated recoverin changes conformation and becomes hydrated at large extents of adsorption as well as to estimate the orientation of myristoylated recoverin with respect to the monolayer plane. In addition, adsorption measurements and electrophoresis of trypsin-treated myristoylated recoverin in the presence of zinc or calcium demonstrated that recoverin has a different conformation but a similar extent of monolayer binding in the presence of such ions.

Characterization of the myristoyl lipid modification of membrane-bound GCAP-2 by 2H solid-state NMR spectroscopy

Guanylate cyclase-activating protein-2 (GCAP-2) is a retinal Ca2+ sensor protein. It is responsible for the regulation of both isoforms of the transmembrane photoreceptor guanylate cyclase, a key enzyme of vertebrate phototransduction. GCAP-2 is N-terminally myristoylated and full activation of its target proteins requires the presence of this lipid modification. The structural role of the myristoyl moiety in the interaction of GCAP-2 with the guanylate cyclases and the lipid membrane is currently not well understood. In the present work, we studied the binding of Ca2+-free myristoylated and non-myristoylated GCAP-2 to phospholipid vesicles consisting of dimyristoylphosphatidylcholine or of a lipid mixture resembling the physiological membrane composition by a biochemical binding assay and 2H solid-state NMR. The NMR results clearly demonstrate the full-length insertion of the aliphatic chain of the myristoyl group into the membrane. Very similar geometrical parameters were determined from the 2H NMR spectra of the myristoyl group of GCAP-2 and the acyl chains of the host membranes, respectively. The myristoyl chain shows a moderate mobility within the lipid environment, comparable to the acyl chains of the host membrane lipids. This is in marked contrast to the behavior of other lipid-modified model proteins. Strikingly, the contribution of the myristoyl group to the free energy of membrane binding of GCAP-2 is only on the order of -0.5 kJ/mol, and the electrostatic contribution is slightly unfavorable, which implies that the main driving forces for membrane localization arises through other, mainly hydrophobic, protein side chain-lipid interactions. These results suggest a role of the myristoyl group in the direct interaction of GCAP-2 with its target proteins, the retinal guanylate cyclases.

Organization, structure and activity of proteins in monolayers

Many different processes take place at the cell membrane interface. Indeed, for instance, ligands bind membrane proteins which in turn activate peripheral membrane proteins, some of which are enzymes whose action is also located at the membrane interface. Native cell membranes are difficult to use to gain information on the activity of individual proteins at the membrane interface because of the large number of different proteins involved in membranous processes. Model membrane systems, such as monolayers at the air-water interface, have thus been extensively used during the last 50 years to reconstitute proteins and to gain information on their organization, structure and activity in membranes. In the present paper, we review the recent work we have performed with membrane and peripheral proteins as well as enzymes in monolayers at the air-water interface. We show that the structure and orientation of gramicidin has been determined by combining different methods. Furthermore, we demonstrate that the secondary structure of rhodopsin and bacteriorhodopsin is indistinguishable from that in native membranes when appropriate conditions are used. We also show that the kinetics and extent of monolayer binding of myristoylated recoverin is much faster than that of the nonmyristoylated form and that this binding is highly favored by the presence polyunsaturated phospholipids. Moreover, we show that the use of fragments of RPE65 allow determine which region of this protein is most likely involved in membrane binding. Monomolecular films were also used to further understand the hydrolysis of organized phospholipids by phospholipases A2 and C.

Recoverin as a redox-sensitive protein

Recoverin is a member of the neuronal calcium sensor (NCS) family of EF-hand calcium binding proteins. In a visual cycle of photoreceptor cells, recoverin regulates activity of rhodopsin kinase in a Ca2+-dependent manner. Like all members of the NSC family, recoverin contains a conserved cysteine (Cys38) in nonfunctional EF-hand 1. This residue was shown to be critical for activation of target proteins in some members of the NCS family but not for interaction of recoverin with rhodopsin kinase. Spectrophotometric titration of Ca2+-loaded recoverin gave 7.6 for the pKa value of Cys38 thiol, suggesting partial deprotonation of the thiol in vivo conditions. An ability of recoverin to form a disulfide dimer and thiol-oxidized monomer under mild oxidizing conditions was found using SDS-PAGE in reducing and nonreducing conditions and Ellman's test. Both processes are reversible and modulated by Ca2+. Although formation of the disulfide dimer takes place only for Ca2+-loaded recoverin, accumulation of the oxidized monomer proceeds more effectively for apo-recoverin. The Ca2+ modulated susceptibility of the recoverin thiol to reversible oxidation may be of potential importance for functioning of recoverin in photoreceptor cells.

Monounsaturated fatty acid modification of Wnt protein: its role in Wnt secretion

The secretion and extracellular transport of Wnt protein are thought to be well-regulated processes. Wnt is known to be acylated with palmitic acid at a conserved cysteine residue (Cys77 in murine Wnt-3a), and this residue appears to be required for the control of extracellular transport. Here, we show that murine Wnt-3a is also acylated at a conserved serine residue (Ser209). Of note, we demonstrated that this residue is modified with a monounsaturated fatty acid, palmitoleic acid. Wnt-3a defective in acylation at Ser209 is not secreted from cells in culture or in Xenopus embryos, but it is retained in the endoplasmic reticulum (ER). Furthermore, Porcupine, a protein with structural similarities to membrane-bound O-acyltransferases, is required for Ser209-dependent acylation, as well as for Wnt-3a transport from the ER for secretion. These results strongly suggest that Wnt protein requires a particular lipid modification for proper intracellular transport during the secretory process.

Structural basis for calcium-induced inhibition of rhodopsin kinase by recoverin

Recoverin, a member of the neuronal calcium sensor branch of the EF-hand superfamily, serves as a calcium sensor that regulates rhodopsin kinase (RK) activity in retinal rod cells. We report here the NMR structure of Ca(2+)-bound recoverin bound to a functional N-terminal fragment of rhodopsin kinase (residues 1-25, called RK25). The overall main-chain structure of recoverin in the complex is similar to structures of Ca(2+)-bound recoverin in the absence of target (<1.8A root-mean-square deviation). The first eight residues of recoverin at the N terminus are solvent-exposed, enabling the N-terminal myristoyl group to interact with target membranes, and Ca(2+) is bound at the second and third EF-hands of the protein. RK25 in the complex forms an amphipathic helix (residues 4-16). The hydrophobic face of the RK25 helix (Val-9, Val-10, Ala-11, Ala-14, and Phe-15) interacts with an exposed hydrophobic groove on the surface of recoverin lined by side-chain atoms of Trp-31, Phe-35, Phe-49, Ile-52, Tyr-53, Phe-56, Phe-57, Tyr-86, and Leu-90. Residues of recoverin that contact RK25 are highly conserved, suggesting a similar target binding site structure in all neuronal calcium sensor proteins. Site-specific mutagenesis and deletion analysis confirm that the hydrophobic residues at the interface are necessary and sufficient for binding. The recoverin-RK25 complex exhibits Ca(2+)-induced binding to rhodopsin immobilized on concanavalin-A resin. We propose that Ca(2+)-bound recoverin is bound between rhodopsin and RK in a ternary complex on rod outer segment disk membranes, thereby blocking RK interaction with rhodopsin at high Ca(2+).

Tuning of a neuronal calcium sensor

Recoverin is a Ca(2+)-regulated signal transduction modulator expressed in the vertebrate retina that has been implicated in visual adaptation. An intriguing feature of recoverin is a cluster of charged residues at its C terminus, the functional significance of which is largely unclear. To elucidate the impact of this segment on recoverin structure and function, we have investigated a mutant lacking the C-terminal 12 amino acids. Whereas in myristoylated recoverin the truncation causes an overall decrease in Ca(2+) sensitivity, results for the non-myristoylated mutant indicate that the truncation primarily affects the high affinity EF-hand 3. The three-dimensional structure of the mutant has been determined by x-ray crystallography. In addition to significant changes in average coordinates compared with wild-type recoverin, the structure provides strong indication of increased conformational flexibility, particularly in the C-terminal domain. Based on these observations, we propose a novel role of the C-terminal segment of recoverin as an internal modulator of Ca(2+) sensitivity.

Recoverin binds exclusively to an amphipathic peptide at the N terminus of rhodopsin kinase, inhibiting rhodopsin phosphorylation without affecting catalytic activity of the kinase

Recoverin is a calcium-dependent inhibitor of rhodopsin kinase. It prevents premature phosphorylation of rhodopsin until the opening of cGMP-gated ion channels causes a decrease in intracellular calcium levels, signaling completion of the light response. This calcium depletion causes release of recoverin from rhodopsin kinase, freeing the kinase to phosphorylate rhodopsin and to terminate the light response. Previous studies have shown that recoverin is able to bind to a region at the N terminus of rhodopsin kinase. In this study we map this interaction interface, showing that residues 1-15 of the kinase form the interaction site for recoverin binding. Mutation of hydrophobic residues in this region have the greatest effect on the interaction. The periodic nature of these residues suggests that they lie along one face of an amphipathic helix. We show that this region is essential for recoverin binding, as a catalytically active kinase lacking these residues is unable to bind recoverin. In addition, we show that neither the N-terminal deletion nor the presence of recoverin inhibits the overall catalytic activity of the kinase, as measured by light-independent autophosphorylation. Finally, we observe that a kinase mutant lacking the N-terminal recoverin binding site is unable to phosphorylate light-activated rhodopsin. Taken together, these data support a model in which recoverin prevents rhodopsin phosphorylation by sterically blocking a region of kinase essential for its interaction with rhodopsin, thereby preventing recognition of rhodopsin as a kinase substrate.

Single-step purification of myristoylated and nonmyristoylated recoverin and substrate dependence of myristoylation level

Recoverin is cotranslationally modified by the covalent linkage of a myristoyl group to its N terminus. It is a member of a family of Ca(2+)-myristoyl switch proteins. Recombinant myristoylated revoverin is currently produced by the cotransformation of bacteria with recoverin and an enzyme that allows N-myristoylation and by supplementing the culture medium with myristic acid. A large variation in the myristoylation level of recoverin and in the amount of myristic acid supplied to the culture medium can be found in the literature. Moreover, although it is known to strongly affect bacterial growth, the amount of ethanol used to solubilize myristic acid is only scarcely mentioned. To improve our understanding of the parameters responsible for recombinant recoverin myristoylation, the effects of myristic acid and ethanol on recoverin myristoylation and expression levels have been systematically studied. In addition, a single-step purification procedure to produce purified myristoylated and nonmyristoylated recombinant recoverin has also been devised. Finally, sodium myristate has been used as an efficient alternative substrate to achieve high myristoylation and expression levels of recoverin. Given that a large number of proteins are myristoylated, these procedures could be applied to several other proteins in addition to recoverin.

Preparation and characteristics of Ca2+-dependent monoclonal antibodies to recoverin

Thirty-four primary hybridoma clones were prepared which expressed monoclonal antibodies to the Ca(2+)-binding protein recoverin. Among the resulting monoclonal antibodies, two Ca(2+)-dependent clones (mAb3 and mAb19) recognizing recoverin were detected by solid-phase immunoenzyme assay. In the presence of Ca(2+), antibodies of the mAb3 and mAb19 clones bound to recoverin several times better than in the absence of Ca(2+). The mAb3 and mAb19 antibodies recognized epitopes located inside the sequences Pro61-Met91 and Pro57-Tyr64 of the recoverin molecule, respectively. The possible mechanism of the Ca(2+)-dependent recognition of recoverin by the prepared monoclonal antibodies is discussed.

Calcium binding sequences in calmyrin regulates interaction with presenilin-2

Calmyrin is a myristoylated calcium binding protein that contains four putative EF-hands. Calmyrin interacts with a number of proteins, including presenilin-2 (PS2). However, the biophysical properties of calmyrin, and the molecular mechanisms that regulate its binding to different partners, are not well understood. By site-directed mutagenesis and Ca2+ binding studies, we found that calmyrin binds two Ca2+ ions with a dissociation constant of approximately 53 microM, and that the two C-terminal EF-hands 3 and 4 bind calcium. Using ultraviolet spectroscopy, circular dichroism (CD), and NMR, we found that Ca(2+)-free and -bound calmyrin have substantially different protein conformations. By yeast two-hybrid assays, we found that both EF-hands 3 and 4 of calmyrin must be intact for calmyrin to interact with PS2-loop sequences. Pulse-chase studies of HeLa cells transfected with calmyrin expression constructs indicated that wild-type (Wt) calmyrin has a half-life of approximately 75 min, whereas a mutant defective in myristoylation turns over more rapidly (half-life of 35 min). By contrast, the half-lives of calmyrin mutants with a disrupted EF-hand 3 or EF-hand 4 were 52 and 170 min, respectively. Using immunofluorescence staining of HeLa cells transfected with Wt and mutant calmyrin cDNAs, we found that both calcium binding and myristoylation are important for dynamic intracellular targeting of calmyrin. Double immunofluorescence microscopy indicated that Wt and myristoylation-defective calmyrin proteins colocalize efficiently and to the same extent with PS2, whereas calmyrin mutants defective in calcium binding display less colocalization with PS2. Our results suggest that calmyrin functions as a calcium sensor and that calcium binding sequences in calmyrin are important for interaction with the PS2 loop.

Cancer-related diseases of the eye: the role of calcium and calcium-binding proteins

The eye provides unique opportunities to study complex biochemical pathways and to describe how components of these pathways contribute to the molecular basis of disease. In this article, the role of calcium-binding proteins in cancer-related diseases of the eye is reviewed. First, paraneoplastic syndromes, or so-called remote effects of cancer, arise from damage to tissues distant from any tumor or its metastases. Many of these syndromes are believed to be immune-mediated. Cancer-associated retinopathy (CAR), a blinding disease due to the degeneration of retinal photoreceptor cells, is one of the best characterized of the paraneoplastic syndromes. The CAR autoantigen has been identified as recoverin, a calcium-binding protein of the EF-hand superfamily. Its features as a calcium-binding protein, along with its function in photoreceptor cells and its role as the CAR autoantigen, are discussed. Next, unlike visual symptoms instigated by a distant tumor, ocular melanoma is the primary malignancy originating in the eye. ALG-2 encodes a pro-apoptotic calcium-binding protein that is down-regulated in ocular melanoma, thus providing these tumor cells with a selective advantage. In addition to background discussion of ALG-2, data describing the expression, cellular localization, and dimerization characteristics of ALG-2 in melanoma cells are presented. Biochemical studies of ALG-2 and its interactions with its target Alix/AIP1 also are presented. Finally, the function of ALG-2 in calcium-induced cell death is discussed. Additional calcium-binding proteins in retina and in ocular tumors are described in relation to different disease entities. Such proteins and their expression in the eye provide valuable examples bridging studies of protein chemistry, cellular function, and human disease.

Recoverin and rhodopsin kinase

The majority of proteins involved in vertebrate phototransduction are expressed specifically in photoreceptors. Recoverin and rhodopsin kinase are expressed primarily in retinal photoreceptors and they interact with each other in a Ca2+-dependent manner. This Ca2+-dependent interaction has been studied extensively in vitro. Experiments utilizing animal models and electrophysiological approaches have started to provide important insight regarding its invivo function. Recoverin can be viewed as a negative regulator of rhodopsin kinase in vertebrate phototransduction. This interaction imparts a negative feedback loop at the receptor level and may play an important role in light adaptation and in recovery.