Multiple determinants direct the orientation of signal-anchor proteins: the topogenic role of the hydrophobic signal domain

Theileria parva 104 kDa microneme--rhoptry protein is membrane-anchored by a non-cleaved amino-terminal signal sequence for entry into the endoplasmic reticulum

The 104 kDa microneme-rhoptry protein (p104) is the only known apical complex organelle-specific protein of Theileria parva. p104 exhibits striking structural similarities to circumsporozoite protein and sporozoite surface protein 2 of Plasmodium yoelii. Their primary sequences contain two hydrophobic segments, located at the amino-and the carboxy-terminus. The p104 amino-terminal hydrophobic region was suggested to be a signal peptide for entry into the endoplasmic reticulum and the extreme carboxy-terminal region to function as a membrane anchor. We have studied the biogenesis of p104 in a cell-free expression system and found that p104 is co-translationally transported into membranes derived from endoplasmic reticulum. The amino-terminal signal peptide is not cleaved off and anchors the protein in the membrane with the carboxy-terminal portion translocated into the lumen. We suggest that in vivo p104 is co-translationally integrated into the membrane of the endoplasmic reticulum, from where it is further transported to the microneme-rhoptry complex. Thus, p104 appears to be a suitable marker to study the development of micronemes and rhoptries in T. parva.

The glycosylation and orientation in the membrane of the third cytoplasmic loop of human P-glycoprotein is affected by mutations and substrates

Multiple topologies have been detected for the COOH-terminal half of the human multidrug resistance P-glycoprotein (P-gp). In one topology, the predicted third cytoplasmic loop (CL3) is on the cytoplasmic side (P-gp-CL3-cyt) of the membrane. In an alternate topology, CL3 is on the extracellular side of the membrane (P-gp-CL3-ext). It is not known if both forms of P-gp are active because it is difficult to distinguish either topology in the full-length molecule. When the halves of P-gp are expressed as separate polypeptides, the two topologies of the C-Half are readily distinguished on SDS-PAGE, because only the C-Half (CL3-ext) is glycosylated. To test whether both topologies can fold into an active enzyme, we assayed for interaction between the N- and C-Halves of P-gp since functional P-gp requires interaction between both halves. In a mutant P-gp (E875C) that gave about equal amounts of both topologies, only the C-Half (CL3-cyt) could be recovered by nickel chromatography after coexpression with the histidine-tagged N-Half P-gp. The isolated N-Half and E875C C-Half (CL3-cyt) polypeptides, when expressed together, exhibited verapamil- and vinblastine-stimulated ATPase activities that were similar to the wild-type enzyme. We also found that biosynthesis of mutant E875C C-Half in the presence of the N-Half P-gp resulted in enhanced expression of C-Half (CL3-cyt). By contrast, interaction of C-Half (CL3-ext) with N-Half P-gp was not detected. These results show that the topology of the C-Half portion of P-gp greatly influences its interactions with the amino-terminal half of the molecule.

Identification of a suppressor of the Dictyostelium profilin-minus phenotype as a CD36/LIMP-II homologue

Profilin is an ubiquitous G-actin binding protein in eukaryotic cells. Lack of both profilin isoforms in Dictyostelium discoideum resulted in impaired cytokinesis and an arrest in development. A restriction enzyme-mediated integration approach was applied to profilin-minus cells to identify suppressor mutants for the developmental phenotype. A mutant with wild-type-like development and restored cytokinesis was isolated. The gene affected was found to code for an integral membrane glycoprotein of a predicted size of 88 kD containing two transmembrane domains, one at the NH2 terminus and the other at the COOH terminus. It is homologous to mammalian CD36/LIMP-II and represents the first member of this family in D. discoideum, therefore the name DdLIMP is proposed. Targeted disruption of the lmpA gene in the profilin-minus background also rescued the mutant phenotype. Immunofluorescence revealed a localization in vesicles and ringlike structures on the cell surface. Partially purified DdLIMP bound specifically to PIP2 in sedimentation and gel filtration assays. A direct interaction between DdLIMP and profilin could not be detected, and it is unclear how far upstream in a regulatory cascade DdLIMP might be positioned. However, the PIP2 binding of DdLIMP points towards a function via the phosphatidylinositol pathway, a major regulator of profilin.

Hydrophobic mismatch between proteins and lipids in membranes

This review addresses the possible consequences of a mismatch in length between the hydrophobic part of membrane-spanning proteins and the hydrophobic bilayer thickness for membrane structure and function. Overviews are given first of the results of studies in defined model systems. These studies address effects of mismatch on protein activity, stability, orientation, aggregational state, localization, and conformation. With respect to the lipids, effects of mismatch are discussed on lipid chain order, phase transition temperature, lipid phase behavior, and microdomain formation. From these studies, it is concluded that hydrophobic mismatch can strongly affect protein and lipid organization, but that the precise consequences depend on the individual properties of the proteins and lipids. Examples of these properties include the propensity of lipids to form non-lamellar structures, the amino acid composition of the hydrophobic transmembrane segments of the proteins, the nature of the membrane anchoring residues, and the number of transmembrane helices. Finally, the effects of mismatch in biological membranes are discussed and its possible consequences for functional membrane processes, such as protein sorting, protein insertion, and regulation of bilayer thickness.

Signal sequences: more than just greasy peptides

Export signal sequences target newly synthesized proteins to the endoplasmic reticulum of eukaryotic cells and the plasma membrane of bacteria. All signal sequences contain a hydrophobic core region, but, despite this, they show great variation in both overall length and amino acid sequence. Recently, it has become clear that this variation allows signal sequences to specify different modes of targeting and membrane insertion and even to perform functions after being cleaved from the parent protein. This review argues that signal sequences are not simply greasy peptides but sophisticated, multipurpose peptides containing a wealth of functional information.

Forced transmembrane orientation of hydrophilic polypeptide segments in multispanning membrane proteins

In a current model of integration of multispanning membrane proteins into the endoplasmic reticulum, it is proposed that the transmembrane segments show alternating translocation initiation and stop-transfer functions. Here, we present evidence for a mode of cotranslational insertion in which an internal signal-anchor sequence with Nexo/Ccyt topology confers a transmembrane disposition onto a preceding hydrophilic segment, resulting in a topology where the hydrophilic segment apparently can slip back and forth across the membrane. Our results demonstrate that hydrophobicity is not, as hitherto thought, an absolute requirement for the formation of a transmembrane segment, and suggest that integral membrane proteins may contain hydrophilic transmembrane segments with a considerable freedom to move in relation to the membrane.

Transmembrane protein insertion orientation in yeast depends on the charge difference across transmembrane segments, their total hydrophobicity, and its distribution

The determinants of transmembrane protein insertion orientation at the endoplasmic reticulum have been investigated in Saccharomyces cerevisiae using variants of a Type III (naturally exofacial N terminus (Nexo)) transmembrane fusion protein derived from the N terminus of Ste2p, the alpha-factor receptor. Small positive and negative charges adjacent to the transmembrane segment had equal and opposite effects on orientation, and this effect was independent of N- or C-terminal location, consistent with a purely electrostatic interaction with response mechanisms. A 3:1 bias toward Nexo insertion, observed in the absence of a charge difference, was shown to reflect the Nexo bias conferred by longer transmembrane segments. Orientation correlated best with total hydrophobicity rather than length, but it was also strongly affected by the distribution of hydrophobicity within the transmembrane segment. The most hydrophobic terminus was preferentially translocated. Insertion orientation thus depends on integration of responses to at least three parameters: charge difference across a transmembrane segment, its total hydrophobicity, and its hydrophobicity gradient. Relative signal strengths were estimated, and consequences for topology prediction are discussed. Responses to transmembrane sequence may depend on protein-translocon interactions, but responses to charge difference may be mediated by the electrostatic field provided by anionic phospholipids.

The endoplasmic reticulum of plant cells and its role in protein maturation and biogenesis of oil bodies

The endoplasmic reticulum (ER) is the port of entry of proteins into the endomembrane system, and it is also involved in lipid biosynthesis and storage. This organelle contains a number of soluble and membrane-associated enzymes and molecular chaperones, which assist the folding and maturation of proteins and the deposition of lipid storage compounds. The regulation of translocation of proteins into the ER and their subsequent maturation within the organelle have been studied in detail in mammalian and yeast cells, and more recently also in plants. These studies showed that in general the functions of the ER in protein synthesis and maturation have been highly conserved between the different organisms. Yet, the ER of plants possesses some additional functions not found in mammalian and yeast cells. This compartment is involved in cell to cell communication via the plasmodesmata, and, in specialized cells, it serves as a storage site for proteins. The plant ER is also equipped with enzymes and structural proteins which are involved in the process of oil body biogenesis and lipid storage. In this review we discuss the components of the plant ER and their function in protein maturation and biogenesis of oil bodies. Due to the large number of cited papers, we were not able to cite all individual references and in many cases we refer the readers to reviews and references therein. We apologize to the authors whose references are not cited.

Mechanism involved in generating the carboxyl-terminal half topology of P-glycoprotein

P-Glycoprotein (Pgp) is a polytopic membrane protein that consists of a tandem repeat of a transmembrane (TM) domain followed by a nucleotide-binding domain. For the carboxyl-terminal half (C-half) of Pgp, at least three different topological orientations have been observed. One major difference between these topologies is reflected in the membrane insertion property of TM8, which is predicted to (1) function as a stop-transfer sequence, (2) lack stop-transfer activity, or (3) function as a signal-anchor sequence. To understand the mechanism involved in generating multiple topological forms for the C-half of Pgp, we investigated the membrane insertion properties of TM segments using the Chinese hamster pgp1 Pgp as a model protein in a cell-free system. We found that TM8 alone or in the presence of TM7 functions as a signal-anchor sequence to insert into membranes with a cytoplasmic amino terminus and an extra-cytoplasmic carboxyl terminus. However, TM8 displayed stop-transfer activity when linked to the C-terminal end of the signal-anchor sequence, TM1. In addition, the membrane orientation of TM8 was found to be regulated by the charge distribution flanking TM8. Interestingly, we found that mammalian and wheat germ ribosomes differentially regulate the signal-anchor and stop-transfer properties of TM8. We conclude that the unique topogenic properties of TM8 direct the generation of multiple C-half topological orientations.

The role of the hydrophobic domain in orienting natural signal sequences within the ER membrane

The orientation of signal sequences during insertion into the endoplasmic reticulum membrane is largely determined by the charged residues flanking the apolar domain. Using recombinant and mutant proteins, also length and hydrophobicity of the apolar segment were shown to affect the orientation: translocation of the N-terminus was found to be favored by long hydrophobic sequences, and translocation of the C-terminus, by short ones. Here, we tested the physiological significance of this phenomenon by mutagenesis of the hydrophobic portion of two natural signals with unusual flanking charges. Extending the hydrophobic domain of the short, cleaved Ncyt/Cexo signal of pre-provasopressin-neurophysin II and shortening that of the Nexo/Ccyt signal anchor of microsomal epoxide hydrolase resulted in a significant fraction of polypeptides inserting in the opposite orientation to that of the wild-type proteins. The topogenic contribution of the hydrophobic domain is thus important for the correct and uniform orientation of natural proteins and can explain the behavior of some of the signals with unusual flanking charges.

The proton motive force, acting on acidic residues, promotes translocation of amino-terminal domains of membrane proteins when the hydrophobicity of the translocation signal is low

We have shown previously that the first transmembrane segment of leader peptidase can function to translocate the polar amino-terminal Pf3 domain across the membrane into the periplasm independently of the proton motive force (pmf) (Lee, J. I., Kuhn, A., and Dalbey, R. E. (1992) J. Biol. Chem. 267, 938-943). We now show that when the first transmembrane segment lacks a strong hydrophobic character, the pmf is required for translocation. In addition, we find that the amino-terminal acidic residue proximal to the transmembrane domain plays a critical role in pmf-dependent amino-terminal translocation. Moreover, the pmf is required to hold the amino-terminal domain in the periplasm to prevent it from slipping such that the amino terminus is no longer exposed to the periplasm. In all cases, translocation occurs under conditions in which the function of the Sec machinery is impaired. These studies show that the low hydrophobicity of the first apolar domain (the translocation signal) can be compensated for by a negative charge in the amino-terminal region, upon which the pmf acts.

Genome-wide analysis of integral membrane proteins from eubacterial, archaean, and eukaryotic organisms

We have carried out detailed statistical analyses of integral membrane proteins of the helix-bundle class from eubacterial, archaean, and eukaryotic organisms for which genome-wide sequence data are available. Twenty to 30% of all ORFs are predicted to encode membrane proteins, with the larger genomes containing a higher fraction than the smaller ones. Although there is a general tendency that proteins with a smaller number of transmembrane segments are more prevalent than those with many, uni-cellular organisms appear to prefer proteins with 6 and 12 transmembrane segments, whereas Caenorhabditis elegans and Homo sapiens have a slight preference for proteins with seven transmembrane segments. In all organisms, there is a tendency that membrane proteins either have many transmembrane segments with short connecting loops or few transmembrane segments with large extra-membraneous domains. Membrane proteins from all organisms studied, except possibly the archaeon Methanococcus jannaschii, follow the so-called "positive-inside" rule; i.e., they tend to have a higher frequency of positively charged residues in cytoplasmic than in extra-cytoplasmic segments.

The role of the ribosome-translocon complex in translation and assembly of polytopic membrane proteins

Newly synthesized polytopic membrane proteins and secretory proteins often share the same target membrane as their primary destination, and in some cases, the cellular machinery that targets and transfers them into or across the membrane. Unlike secretory proteins, which are localized to the external compartment, each polytopic membrane protein molecule must be partitioned among the cytoplasm, the membrane and the external milieu. How does the ribosome-translocon complex cope with the different domains of polytopic membrane proteins?