Aberrant membrane insertion of a cytoplasmic tail deletion mutant of the hemagglutinin-neuraminidase glycoprotein of Newcastle disease virus

Morbillivirus and henipavirus attachment protein cytoplasmic domains differently affect protein expression, fusion support and particle assembly

The amino-terminal cytoplasmic domains of paramyxovirus attachment glycoproteins include trafficking signals that influence protein processing and cell surface expression. To characterize the role of the cytoplasmic domain in protein expression, fusion support and particle assembly in more detail, we constructed chimeric Nipah virus (NiV) glycoprotein (G) and canine distemper virus (CDV) haemagglutinin (H) proteins carrying the respective heterologous cytoplasmic domain, as well as a series of mutants with progressive deletions in this domain. CDV H retained fusion function and was normally expressed on the cell surface with a heterologous cytoplasmic domain, while the expression and fusion support of NiV G was dramatically decreased when its cytoplasmic domain was replaced with that of CDV H. The cell surface expression and fusion support functions of CDV H were relatively insensitive to cytoplasmic domain deletions, while short deletions in the corresponding region of NiV G dramatically decreased both. In addition, the first 10 residues of the CDV H cytoplasmic domain strongly influence its incorporation into virus-like particles formed by the CDV matrix (M) protein, while the co-expression of NiV M with NiV G had no significant effect on incorporation of G into particles. The cytoplasmic domains of both the CDV H and NiV G proteins thus contribute differently to the virus life cycle.

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.

Membrane assembly of the triple-spanning coronavirus M protein. Individual transmembrane domains show preferred orientation

The M protein of mouse hepatitis virus strain A59 is a triple-spanning membrane protein which assembles with an uncleaved internal signal sequence, adopting an NexoCcyt orientation. To study the insertion mechanism of this protein, domains potentially involved in topogenesis were deleted and the effects analyzed in topogenesis were deleted and the effects analyzed in several ways. Mutant proteins were synthesized in a cell-free translation system in the presence of microsomal membranes, and their integration and topology were determined by alkaline extraction and by protease-protection experiments. By expression in COS-1 and Madin-Darby canine kidney-II cells, the topology of the mutant proteins was also analyzed in vivo. Glycosylation was used as a biochemical marker to assess the disposition of the NH2 terminus. An indirect immunofluorescence assay on semi-intact Madin-Darby canine kidney-II cells using domain-specific antibodies served to identify the cytoplasmically exposed domains. The results show that each membrane-spanning domain acts independently as an insertion and anchor signal and adopts an intrinsic preferred orientation in the lipid bilayer which corresponds to the disposition of the transmembrane domain in the wild-type assembled protein. These observations provide further insight into the mechanism of membrane integration of multispanning proteins. A model for the insertion of the coronavirus M protein is proposed.

Membrane assembly of the triple-spanning coronavirus M protein. Individual transmembrane domains show preferred orientation

The M protein of mouse hepatitis virus strain A59 is a triple-spanning membrane protein which assembles with an uncleaved internal signal sequence, adopting an NexoCcyt orientation. To study the insertion mechanism of this protein, domains potentially involved in topogenesis were deleted and the effects analyzed in topogenesis were deleted and the effects analyzed in several ways. Mutant proteins were synthesized in a cell-free translation system in the presence of microsomal membranes, and their integration and topology were determined by alkaline extraction and by protease-protection experiments. By expression in COS-1 and Madin-Darby canine kidney-II cells, the topology of the mutant proteins was also analyzed in vivo. Glycosylation was used as a biochemical marker to assess the disposition of the NH2 terminus. An indirect immunofluorescence assay on semi-intact Madin-Darby canine kidney-II cells using domain-specific antibodies served to identify the cytoplasmically exposed domains. The results show that each membrane-spanning domain acts independently as an insertion and anchor signal and adopts an intrinsic preferred orientation in the lipid bilayer which corresponds to the disposition of the transmembrane domain in the wild-type assembled protein. These observations provide further insight into the mechanism of membrane integration of multispanning proteins. A model for the insertion of the coronavirus M protein is proposed.

A zein signal sequence functions as a signal-anchor when fused to maize alcohol dehydrogenase

A chimeric gene, preZad, was constructed encoding a zein signal sequence fused precisely to the amino terminus of maize alcohol dehydrogenase 1. Translocation and processing of this chimeric preZad protein were assayed in vitro using a rabbit reticulocyte lysate translation system supplemented with canine pancreatic microsomes. PreZad was cotranslationally translocated across the vesicular membranes. Unexpectedly, the signal sequence was not removed although a suitable cleavage site was preserved and presented within the vesicle lumen. Failure to cleave the signal sequence was apparently not due to the lack of a beta-turn near the processing site. When a beta-turn was introduced near the cleavage site through site-directed mutagenesis, no processing was observed. PreZad was not solubilized by alkaline treatment of the microsomes, indicating an integral membrane association. Resistance to proteolysis, in the absence of detergent, indicates that preZad is associated with the membranes in a type II orientation (C-terminus in and N-terminus outside the vesicles). Analysis of truncated versions of preZad showed that it is the uncleaved signal sequence that functions as a signal-anchor. Changing the ratio of net charge flanking the signal sequence to less than 1 (N-terminal:C-terminal) did not alter the type II membrane orientation, as would have been predicted by the 'positive-in rule'. Our results provide additional insight into the role of the passenger protein and signal sequence-flanking regions in recognition of a signal peptidase processing site, and the orientation of insertion of a signal-anchor sequence into the endoplasmic reticulum membrane.