Dissecting the physiological role of selective transmembrane-segment retention at the ER translocon

The membrane integration of polytopic proteins is coordinated at the endoplasmic reticulum (ER) by the conserved Sec61 translocon, which facilitates the lateral release of transmembrane (TM) segments into the lipid phase during polypeptide translocation. Here we use a site-specific crosslinking strategy to study the membrane integration of a new model protein and show that the TM segments of the P2X2 receptor are retained at the Sec61 complex for the entire duration of the biosynthetic process. This extremely prolonged association implicates the Sec61 complex in the regulation of the membrane integration process, and we use both in vitro and in vivo analyses to study this effect further. TM-segment retention depends on the association of the ribosome with the Sec61 complex, and complete lateral exit of the P2X2 TM segments was only induced by the artificial termination of translation. In the event of the premature release of P2X2 TM1 from the ER translocon, the truncated polypeptide fragment was to found aggregate in the ER membrane, suggesting a distinct physiological requirement for the delayed release of TM segments from the ER translocon site.

Functional analysis of the transmembrane domain in paramyxovirus F protein-mediated membrane fusion

To enter cells, enveloped viruses use fusion-mediating glycoproteins to facilitate the merger of the viral and host cell membranes. These glycoproteins undergo large-scale irreversible refolding during membrane fusion. The paramyxovirus parainfluenza virus 5 mediates membrane merger through its fusion protein (F). The transmembrane (TM) domains of viral fusion proteins are typically required for fusion. The TM domain of F is particularly interesting in that it is potentially unusually long; multiple calculations suggest a TM helix length between 25 and 48 residues. Oxidative cross-linking of single-cysteine substitutions indicates the F TM trimer forms a helical bundle within the membrane. To assess the functional role of the paramyxovirus parainfluenza virus 5 F protein TM domain, alanine scanning mutagenesis was performed. Two residues located in the outer leaflet of the bilayer are critical for fusion. Multiple amino acid substitutions at these positions indicate the physical properties of the side chain play a critical role in supporting or blocking fusion. Analysis of intermediate steps in F protein refolding indicated that the mutants were not trapped at the open stalk intermediate or the prehairpin intermediate. Incorporation of a known F protein destabilizing mutation that causes a hyperfusogenic phenotype restored fusion activity to the mutants. Further, altering the curvature of the lipid bilayer by addition of oleic acid promoted fusion of the F protein mutants. In aggregate, these data indicate that the TM domain plays a functional role in fusion beyond merely anchoring the protein in the viral envelope and that it can affect the structures and steady-state concentrations of the various conformational intermediates en route to the final postfusion state. We suggest that the unusual length of this TM helix might allow it to serve as a template for formation of or specifically stabilize the lipid stalk intermediate in fusion.

An emerging consensus for the structure of EmrE

The archetypical member of the small multidrug-resistance family is EmrE, a multidrug transporter that extrudes toxic polyaromatic cations from the cell coupled to the inward movement of protons down a concentration gradient. The architecture of EmrE was first defined from the analysis of two-dimensional crystals by cryoelectron microscopy (cryo-EM), which showed that EmrE was an unusual asymmetric dimer formed from a bundle of eight alpha-helices. The most favoured interpretation of the structure was that the monomers were oriented in opposite orientations in the membrane in an antiparallel orientation. A model was subsequently built based upon the cryo-EM data and evolutionary constraints and this model was consistent with mutagenic data indicating which amino-acid residues were important for substrate binding and transport. Two X-ray structures that differed significantly from the cryo-EM structure were subsequently retracted owing to a data-analysis error. However, the revised X-ray structure with substrate bound is extremely similar to the model built from the cryo-EM structure (r.m.s.d. of 1.4 A), suggesting that the proposed antiparallel orientation of the monomers is indeed correct; this represents a new structural paradigm in membrane-protein structures. The vast majority of mutagenic and biochemical data corroborate this structure, although cross-linking studies and recent EPR data apparently support a model of EmrE that contains parallel dimers.

Folding scene investigation: membrane proteins

Investigations into protein folding have concentrated on experimentally tractable proteins with the result that membrane protein folding remains unsolved. New evidence is providing insight into the nature of the interactions stabilising the folded state of alpha-helical membrane proteins as well as giving hints on the character of the folding transition state. These developments show that classical methods used for water-soluble proteins can be successfully adapted for membrane proteins. The advances, coupled with increasing numbers of solved crystal structures, augur well for future research into the mechanisms of membrane protein folding.

Lipid-dependent membrane protein topogenesis

The topology of polytopic membrane proteins is determined by topogenic sequences in the protein, protein-translocon interactions, and interactions during folding within the protein and between the protein and the lipid environment. Orientation of transmembrane domains is dependent on membrane phospholipid composition during initial assembly as well as on changes in lipid composition postassembly. The membrane translocation potential of negative amino acids working in opposition to the positive-inside rule is largely dampened by the normal presence of phosphatidylethanolamine, thus explaining the dominance of positive residues as retention signals. Phosphatidylethanolamine provides the appropriate charge density that permits the membrane surface to maintain a charge balance between membrane translocation and retention signals and also allows the presence of negative residues in the cytoplasmic face of proteins for other purposes.

Transmembrane domain length of viral K+ channels is a signal for mitochondria targeting

K(+) channels operate in the plasma membrane and in membranes of organelles including mitochondria. The mechanisms and topogenic information for their differential synthesis and targeting is unknown. This article describes 2 similar viral K(+) channels that are differentially sorted; one protein (Kesv) is imported by the Tom complex into the mitochondria, the other (Kcv) to the plasma membrane. By creating chimeras we discovered that mitochondrial sorting of Kesv depends on a hierarchical combination of N- and C-terminal signals. Crucial is the length of the second transmembrane domain; extending its C terminus by > or = 2 hydrophobic amino acids redirects Kesv from the mitochondrial to the plasma membrane. Activity of Kesv in the plasma membrane is detected electrically or by yeast rescue assays only after this shift in sorting. Hence only minor structural alterations in a transmembrane domain are sufficient to switch sorting of a K(+) channel between the plasma membrane and mitochondria.