Spore coat protein of Bacillus subtilis. Structure and precursor synthesis.

The coat protein of Bacillus subtilis spores comprises about 10% of the total dry weight of spores and 25% of the total spore protein. One protein with a molecular weight of 13,000 to 15,000 comprises a major portion of the spore coat. This mature spore coat protein has histidine at its NH2 terminus and is relatively rich in hydrophobic amino acids. Netropsin, and antibiotic which binds to A-T-rich regions of DNA and inhibits sporulation, but not growth, decreased the synthesis of this spore coat protein by 75%. A precursor spore coat protein with a molecular weight of 25,000 is made initially at t1 of sporulation and is converted to the mature spore coat protein with a molecular weight of 13,500 at t2 - t3. These data indicate that the spore coat protein gene is expressed very early in sporulation prior to the modifications of RNA polymerase which have been noted.     

The internal signal sequence of Escherichia coli leader peptidase is necessary, but not sufficient, for its rapid membrane assembly

Leader peptidase of Escherichia coli, a protein of 323 residues, has three hydrophobic domains. The first, residues 1-22, is the most apolar and is followed by a polar region (23-61) which faces the cytoplasm. The second hydrophobic domain (residues 62-76) spans the membrane. The third hydrophobic domain, which has a minimal apolar character, and the polar, carboxyl-terminal two-thirds of the protein are exposed to the periplasm. Deletion of either the amino terminus (residues 4-50) or the third hydrophobic region (residues 83-98) has almost no effect on the rate of leader peptidase membrane assembly, while the second hydrophobic domain is essential for insertion (Dalbey, R., and Wickner, W. (1987) Science 235, 783-787). To further define the roles of these domains, we have replaced the normal, cleaved leader sequence of pro-OmpA and M13 procoat with regions containing either the first or second apolar domain of leader peptidase. The second apolar domain supports the translocation of OmpA or coat protein across the plasma membrane, establishing its identity as an internal, uncleaved signal sequence. In addition to this sequence, we now find that leader peptidase needs either the amino-terminal domain or the third hydrophobic domain to permit its rapid membrane assembly. These results show that, although a signal sequence is necessary for rapid membrane assembly of leader peptidase, it is not sufficient.     

Membrane topography of ColE1 gene products: the hydrophobic anchor of the colicin E1 channel is a helical hairpin.

The paucity of crystallographic data on the structure of intrinsic membrane proteins necessitates the development of additional techniques to probe their structures. The colicin E1 ion channel domain contains one prominent hydrophobic region near its COOH terminus that has been proposed to be an anchor for the assembly of the channel. Saturation site-directed mutagenesis of the hydrophobic anchor region of the colicin E1 ion channel was used to probe whether it spanned the bilayer once or twice. A nonpolar amino acid was replaced by a charged residue in 29 mutations made at 26 positions in the channel domain. Substitution of the charged amino acid at all positions except those in the center of the hydrophobic region and the periphery of the hydrophobic region caused a large decrease in the cytotoxicity of the purified mutant colicin E1 protein. This result implies that the hydrophobic domain spans the membrane bilayer twice in a helical hairpin loop, with the center of this domain residing in an aqueous or polar phase. The lengths of the trans-membrane helices appear to be approximately 18 and 16 residues. The absence of significant changes in ion selectivity in five of nine mutants indicated that these mutations did not cause a large change in the channel structure. The ion selectivity changes in four mutants and those previously documented for the flanking Lys residues imply that the hydrophobic hairpin is part of the channel lumen. Water may "abhor" the hydrophobic side of the channel, explaining the small effects of residue charge changes on ion selectivity.     

Isolation of mutants in M13 coat protein that affect its synthesis, processing, and assembly into phage

The major coat protein (gene 8 protein) of bacteriophage M13 has been studied intensively as a model of membrane assembly, protein packing, and protein-DNA interactions. Because this protein is essential for assembly of the phage, very few mutants have been isolated. We have therefore cloned the gene 8 into a plasmid under control of the araB promoter. In the presence of arabinose, the cloned gene is expressed at a rate comparable to that in an M13-infected cell. Plasmid-derived procoat is inserted across the plasma membrane and processed to coat at a normal rate. The coat can support plaque formation by a defective M13 virus (M13am8) with an amber mutation in its procoat gene. This complementation assay was used to screen the mutagenized, cloned gene 8 for mutants which fail to make fully functional coat. Mutants were obtained which fail to synthesize procoat, which do not convert procoat to mature coat protein, or in which the coat protein is incapable of assembling into infectious virions.     

Mutations in the C-terminal hydrophobic domain of pseudorabies virus gIII affect both membrane anchoring and protein export.

The transmembrane and anchor region of pseudorabies virus gIII is postulated to be in the 35 hydrophobic amino acids (residues 436 to 470) found near the carboxy terminus of the 479-amino-acid envelope protein. In this study, we used a genetic approach to localize the functional gIII membrane anchor between amino acids 443 and 466. Mutant gIII proteins lacking the membrane anchor were not associated with virus particles, indicating that membrane retention is a prerequisite for virion localization. Unexpectedly, the specific hydrophobic gIII sequence defined by these deletions was not required for membrane anchor function since the entire region could be replaced with leucine residues without affecting gIII membrane retention, export, or virion localization. The hydrophobic region appears to encode more than the membrane anchor domain since both efficiency of posttranslational processing and localization to virions are affected by mutations in this region. We speculate that the composition of the hydrophobic domain influences the overall conformation of gIII, which in turn effects the efficiency of gIII export and processing. The virion localization phenotype is probably indirect and reflects the efficiency of protein processing. This conclusion provides insight into the mechanism of glycoprotein incorporation into virions.     

Membrane assembly of M13 major coat protein: evidence for a structural adaptation in the hinge region and a tilted transmembrane domain

New insights into the low-resolution structure of the hinge region and the transmembrane domain of the membrane-bound major coat protein of the bacteriophage M13 are deduced from a single cysteine-scanning approach using fluorescence spectroscopy. New mutant coat proteins are labeled and reconstituted into phospholipid bilayers with varying headgroup compositions (PC, PE, and PG) and thicknesses (14:1PC, 18:1PC, and 22:1PC). Information about the polarity of the local environment around the labeled sites is deduced from the wavelength of maximum emission using AEDANS attached to the SH groups of the cysteines as a fluorescent probe. It is found that the protein is almost entirely embedded in the membrane, whereas the phospholipid headgroup composition of the membrane hardly affects the overall embedment of the protein in the membrane. From the assessment of a hydrophobic and hydrophilic face of the transmembrane helix, it is concluded that the helix is tilted with respect to the membrane normal. As compared to the thicker 18:1PC and 22:1PC membranes, reconstitution of the protein in the thin 14:1PC membranes results in a loss of helical structure and in the formation of a stretched conformation of the hinge region. It is suggested that the hinge region acts as a flexible spring between the N-terminal amphipathic arm and transmembrane hydrophobic helix. On average, the membrane-bound state of the coat protein can be seen as a gently curved and tilted, "banana-shaped" molecule, which is strongly anchored in the membrane-water interface at the C-terminus. From our experiments, we propose a rather small conformational adaptation of the major coat protein as the most likely reversible mechanism for responding to environmental changes during the bacteriophage disassembly and assembly process.     

Recombinant forms of M13 procoat with an OmpA leader sequence or a large carboxy-terminal extension retain their independence of secY function.

The assembly of phage M13 procoat protein into the plasma membrane of Escherichia coli is independent of the secY protein. To test whether this is caused by the unusually small size of procoat, we fused DNA encoding 103 amino acids to the carboxy-terminal end of the procoat gene. The resulting fusion protein, which attains the same membrane-spanning conformation as mature coat protein, still does not require the secY function for membrane assembly. To determine whether the leader sequence governs interaction with the secY protein, we genetically exchanged the leader peptides between procoat and pro-OmpA, a protein which does require secY for its membrane assembly. Each of the resulting hybrid proteins assembles across the plasma membrane, though at a reduced rate. Membrane assembly of the fusion of procoat leader and OmpA required secY function, whereas assembly of the pro-OmpA leader/coat protein fusion was independent of secY. Properties of the entire procoat molecule, rather than its small size or a specific property of its leader peptide determines its mode of membrane assembly.     

Hydrophobic forces drive spontaneous membrane insertion of the bacteriophage Pf3 coat protein without topological control

Bacterial integral inner membrane proteins are either translocated across the lipid bilayer using an energy-driven enzyme, such as the Sec translocase, or they might interact directly with the membrane due to hydrophobic forces. We report that the single-spanning Pf3 coat protein is spontaneously inserted into the membrane of Escherichia coli and requires the electrical component of the membrane potential (DeltaPsi) to translocate its N-terminal region. This results in a final N(out)C(in) orientation of the protein in the cytoplasmic membrane, due the potential-driven translocation of the aspartyl residue at position 18 in the hydrophilic N-terminal tail. Uncharged protein tails are only translocated when the hydrophobic transmembrane region of the protein has been extended. An extended transmembrane anchor allows membrane insertion in the absence of an electrochemical membrane potential, but also causes the loss of a strict determination of the topology.     

Crystal Structure of the Minor Pilin CofB,the Initiator of CFA/III Pilus Assembly in Enterotoxigenic Escherichia coli

Type IV pili are extracellular polymers of the major pilin subunit. These subunits are held together in the pilus filament by hydrophobic interactions among their N-terminal α-helices, which also anchor the pilin subunits in the inner membrane prior to pilus assembly. Type IV pilus assembly involves a conserved group of proteins that span the envelope of Gram-negative bacteria. Among these is a set of minor pilins, so named because they share their hydrophobic N-terminal polymerization/membrane anchor segment with the major pilins but are much less abundant. Minor pilins influence pilus assembly and retraction, but their precise functions are not well defined. The Type IV pilus systems of enterotoxigenic Escherichia coli and Vibrio cholerae are among the simplest of Type IV pilus systems and possess only a single minor pilin. Here we show that the enterotoxigenic E. coli minor pilins CofB and LngB are required for assembly of their respective Type IV pili, CFA/III and Longus. Low levels of the minor pilins are optimal for pilus assembly, and CofB can be detected in the pilus fraction. We solved the 2.0 Å crystal structure of N-terminally truncated CofB, revealing a pilin-like protein with an extended C-terminal region composed of two discrete domains connected by flexible linkers. The C-terminal region is required for CofB to initiate pilus assembly. We propose a model for CofB-initiated pilus assembly with implications for understanding filament growth in more complex Type IV pilus systems as well as the related Type II secretion system.     

NMR structure and molecular dynamics of the in-plane membrane anchor of nonstructural protein 5A from bovine viral diarrhea virus

Hepatitis C virus (HCV) nonstructural protein 5A (NS5A) is a monotopic membrane protein anchored to the membrane by an N-terminal in-plane amphipathic alpha-helix. This membrane anchor is essential for the assembly of a functional viral replication complex. Although amino acid sequences differ considerably, putative membrane anchors with amphipathic features were predicted in NS5A from related Flaviviridae family members, in particular bovine viral diarrhea virus (BVDV), the prototype representative of the genus Pestivirus. We report here the NMR structure of the membrane anchor 1-28 of NS5A from BVDV in the presence of different membrane mimetic media. This anchor includes a long amphipathic alpha-helix of 21 residues interacting in-plane with the membrane interface and including a putative flexible region. Molecular dynamic simulation at a water-dodecane interface used to mimic the surface separating a lipid bilayer and an aqueous medium demonstrated the stability of the helix orientation and the location at the hydrophobic-hydrophilic interface. The flexible region of the helix appears to be required to allow the most favorable interaction of hydrophobic and hydrophilic side chain residues with their respective environment at the membrane interface. Despite the lack of amino acid sequence similarity, this amphipathic helix shares common structural features with that of the HCV counterpart, including a stable, hydrophobic N-terminal segment separated from the more hydrophilic C-terminal segment by a local, flexible region. These structural conservations point toward conserved roles of the N-terminal in-plane membrane anchors of NS5A in replication complex formation of HCV, BVDV, and other related viruses.     

Export and sorting of theEscherichia coli outer membrane protein OmpA

Results of studies, mostly using the outer membrane, 325 residue protein OmpA, are reviewed which concern its translocation across the plasma membrane and incorporation into the outer membrane ofEscherichia coli. For translocation, neither a unique export signal, acting in a positive fashion within the mature part of the precursor, nor a unique conformation of the precursor is required. Rather, the mature part of a secretory protein has to be export-compatible. Export-incompatibility can be caused by a stretch of 16 (but not 8 or 12) hydrophobic residues, too low a size of the polypeptide (smaller than 75 residue precursors), net positive charge at the N-terminus, or lack of a turn potential at the same site. It is not yet clear whether binding sites for chaperonins (SecB, trigger factor, GroEL) within OmpA are importantin vivo. The mechanism of sorting of outer membrane proteins is not yet understood. The membrane part of OmpA, encompassing residues 1 to about 170, it thought to traverse the membrane eight times in antiparallel -sheet conformation. At least the structure of the last -strand (residues 160–170) is of crucial importance for membrane assembly. It must be amphiphilic or hydrophobic, these properties must extend over at least nine residues, and it must not contain a proline residue at or near its center. Membrane incorporation of OmpA involves a conformational change of the protein and it could be that the last -strand initiates folding and assembly in the outer membrane.     

The amino-terminal structures that determine topological orientation of cytochrome P-450 in microsomal membrane.

We previously showed that the amino-terminal region of P-450 is responsible not only for targeting to endoplasmic reticulum (ER) membrane but also for stable anchoring to the membrane. In the present study, we introduced several mutations or deletions into the signal-anchor region of the chimeric proteins in which the amino-terminal regions of two forms of cytochrome P-450 were fused to the mature portion of interleukin 2. The amino-terminal acidic amino acid residues were replaced with basic amino acid residues or the hydrophobic core sequences were partially deleted, and these mutant proteins were assayed in vitro for their capacity to be inserted into or translocated across the ER membrane. The proteins that received the former manipulations were processed and the IL-2 portion was translocated across the membrane. In one case, the processing did not occur, thereby enabling the chimeric protein to anchor on the luminal side of the ER. Those that received the latter manipulation were also processed and the IL-2 portion translocated across the ER. These results strongly suggest that the signal-anchor function is determined both by the amino-terminal charged amino acid residues and by the length of the hydrophobic stretch.     

Nonpolar substitution at the C-terminus of the prion protein, a mimic of the glycosylphosphatidylinositol anchor, partially impairs amyloid fibril formation

In contrast to most amyloidogenic proteins or peptides that do not contain any significant posttranslational modifications, the prion protein (PrP) is modified with either one or two polysaccharides and a GPI anchor which attaches PrP to the plasma membrane. Like other amyloidogenic proteins, however, PrP adopts a fibrillar shape when converted to a disease-specific conformation. Therefore, PrP polymerization offers a unique opportunity to examine the effects of biologically relevant nonpeptidic modifications on conversion to the amyloid conformation. To test the extent to which a long hydrophobic chain at the C-terminus affects the intrinsic amyloidogenic propensity of PrP, we modified recombinant PrP with an N-myristoylamidomaleimidyl group, which can serve as a membrane anchor. We show that while this modification increases the affinity of PrP for the cell membrane, it does not alter the structure of the protein. Myristoylation of PrP affected amyloid formation in two ways: (i) it substantially decreased the extent of fibrillation, presumably due to off-pathway aggregation, and (ii) it prohibited assembly of filaments into higher order fibrils by preventing their lateral association. The negative effect on lateral association was abolished if the myristoylated moiety at the C-terminus was replaced by a polar group of similar size or by a hydrophobic group of smaller size. When preformed PrP fibrils were provided as seeds, myristoylated PrP supported fibril elongation and formation of higher order fibrils composed of several filaments. Our studies illustrate that, despite a bulky hydrophobic moiety at C-terminus, myristoylated PrP can still incorporate into fibrillar structure and that the C-terminal hydrophobic substitution does not affect the size of the proteinase K resistant core but controls the mode of lateral assembly of filaments into higher order fibrils.     

Filamentous Bacterial viruses

The filamentous bacterial virus is a simple and well-characterized model system for studying how genetic information is transformed into molecular machines. The viral DNA is a single-stranded circle coding for about 10 proteins. The major viral coat protein is largely α-helical, with about 46 amino acid residues. Several thousand identical copies of this protein in a helical array form a hollow cylindrical tube 1–2μ long, of outer diameter 60 Å and inner diameter 20 Å, with the twisted circular DNA extending down the core of the tube. Before assembly, the viral coat protein spans the cell membrane, and assembly involves extrusion of the coat from the membrane. X-ray fibre diffraction patterns of the Pf 1 species of virus at 4°C, oriented in a strong magnetic field, give three-dimensional data to 4 Å resolution. An electron density map calculated from native virus and a single iodine derivative, using the maximum entropy technique, shows a helix pitch of 5.9 Å. This may indicate a stretched A-helix, or it may indicate a partially 3₁₀ helix conformation, resulting from the fact that the coat protein is an integral membrane protein before assembly, and is still in the hydrophobic environment of other coat proteins after assembly.     

Alterations in the transport and processing of Rous sarcoma virus envelope glycoproteins mutated in the signal and anchor regions

The env gene of Rous sarcoma virus codes for two glycoproteins which are located on the surface of infectious virions. Subcloning of these coding sequences in the place of the late region of SV40 DNA has allowed the expression of a normally glycosylated, functionally active glycoprotein complex on the surface of monkey cells. Through the use of site-directed mutagenesis, the role of specific amino acids in the signal peptide, signal peptidase cleavage site, and membrane anchor region have been investigated. Amino-terminal mutations have shown that deletion of the signal peptidase cleavage site along with one or two amino acids of the hydrophobic signal peptide results in the synthesis of an unglycosylated, uncleaved, and presumably cytoplasmically located precursor. Nevertheless, changing the signal peptidase cleavage site from ala/asp to ala/asn does not block the translocation of the glycoprotein across the membrane or the action of the peptidase. At the other end of the molecule, carboxy-terminal mutations have shown that the deletion of the hydrophobic membrane anchor region is not sufficient for the secretion of the truncated glycoprotein. Interpretations of these results based on recent models for protein transport and secretion are discussed.     

Cryo-electron tomography of clathrin-coated vesicles: structural implications for coat assembly

Clathrin-coated vesicles mediate vesicular traffic in cells. Three-dimensional image reconstructions of homogenous populations of in vitro assembled clathrin coats have yielded a molecular model for clathrin and its interactions with some of its partners. The intrinsic averaging required for those calculations has precluded detailed analysis of heterogeneous populations of clathrin-coated vesicles isolated from cells. We have therefore used cryo-electron tomography to study the lattice organization of individual clathrin-coated vesicles and the disposition of the captured vesicle with respect to the surrounding coat. We find a wide range of designs for the clathrin lattice, with different patterns of pentagonal, hexagonal, and occasionally heptagonal facets. Many coats, even smaller ones, enclose membrane vesicles, which are generally offset from the center of the clathrin shell. The electron density distribution between the coat and the underlying vesicle is not uniform, and the number of apparent contacts that anchor the clathrin lattice to the vesicle membrane is significantly less than the number of clathrin heavy chains in the assembly. We suggest that the eccentric position of the vesicle reflects the polarity of assembly, from initiation of coat formation to membrane pinching.     

SRP-dependent co-translational targeting and SecA-dependent translocation analyzed as individual steps in the export of a bacterial protein

Recently it has been recognized that the signal recognition particle (SRP) of Escherichia coli represents a specific targeting device for hydrophobic inner membrane proteins. It has remained unclear, however, whether the bacterial SRP functions in concert with SecA, which is required for the translocation of secretory proteins across the inner membrane. Here, we have analyzed a hybrid protein constructed by fusing the signal anchor sequence of an SRP-dependent inner membrane protein (MtlA) to the mature part of an exclusively SecA-requiring secretory protein (OmpA). We show that the signal anchor sequence of MtlA confers the novel properties onto nascent chains of OmpA of being co-translationally recognized and targeted to SecY by SRP. Once targeted to SecY, ribosome-associated nascent chains of the hybrid protein, however, remain untranslocated unless SecA is present. These results indicate that SRP and SecA cooperate in a sequential, non-overlapping manner in the topogenesis of those membrane proteins which, in addition to a signal anchor sequence, harbor a substantial hydrophilic domain to be translocated into the periplasm.     

Anchoring mechanisms of membrane-associated M13 major coat protein

Bacteriophage M13 major coat protein is extensively used as a biophysical, biochemical, and molecular biology reference system for studying membrane proteins. The protein has several elements that control its position and orientation in a lipid bilayer. The N-terminus is dominated by the presence of negatively charged amino acid residues (Glu2, Asp4, and Asp5), which will always try to extend into the aqueous phase and therefore act as a hydrophilic anchor. The amphipathic and the hydrophobic transmembrane part contain the most important hydrophobic anchoring elements. In addition there are specific aromatic and charged amino acid residues in these domains (Phe 11, Tyr21, Tyr24, Trp26, Phe42, Phe45, Lys40, Lys43, and Lys44) that fine-tune the association of the protein to the lipid bilayer. The interfacial Tyr residues are important recognition elements for precise protein positioning, a function that cannot be performed optimally by residues with an aliphatic character. The Trp26 anchor is not very strong: depending on the context, the tryptophan residue may move in or out of the membrane. On the other hand, Lys residues and Phe residues at the C-terminus of the protein act in a unique concerted action to strongly anchor the protein in the lipid bilayer.     

Secretion and membrane integration of a filamentous phage-encoded morphogenetic protein

The filamentous phage-encoded gene IV protein is required at high levels for virus assembly, although it is not a constituent of the virion. It is an integral membrane protein that does not contain an extended hydrophobic region of the kind often required for stable integration in the inner membrane. Rather, like a number of Escherichia coli outer membrane proteins, pIV is rich in charged amino acid residues and is predicted to consist of extensive beta-sheet structures. In phage-producing cells, pIV is primarily detected in the outer membrane, while in cells that produce it from the cloned gene, pIV is found in both the inner and outer membranes. The protein is synthesized as a precursor. Following cleavage of the signal sequence and translocation into the periplasm, the mature form is initially found as a soluble species. Soluble pIV then integrates into the membrane with a half-time of one to two minutes. Neither phage assembly nor other phage proteins are needed for this membrane integration, and phage assembly does not require the presence of the soluble form. The gene IV protein may be part of the structure through which the assembling phage is extruded.