Membrane fusion process of Semliki Forest virus. I: Low pH-induced rearrangement in spike protein quaternary structure precedes virus penetration into cells.

Abstract

Abstract. The Semliki Forest virus (SFV) directs the synthesis of a heterodimeric membrane protein complex which is used for virus membrane assembly during budding at the surface of the infected cell, as well as for low pH-induced membrane fusion in the endosomes when particles enter new host cells . Existing evidence suggests that the El protein subunit carries the fusion potential of the heterodimer, whereas the E2 subunit, or its intracellular precursor p62, is required for binding to the nucleocapsid. We show here that during virus uptake into acidic endosomes the LPHAVIRUSES, such as Semliki Forest virus (SFV)l and Sindbis virus, are enveloped animal viruses A IL which mature by budding at the plasma membrane (PM) ofinfected cells and enter new cells by an acid-induced membrane fusion process inside the endosomal compartment Schlesinger and Schlesinger, 1986). Because of their simple structure and efficient replication these viruses represent useful systems for studying the molecular mechanisms of membrane budding and fusion. SFV directs the synthesis of three structural proteins, the capsid protein and two transmembrane glycoproteins, p62 and El (Garoff et al., 1982). The capsid protein associates in the cell cytoplasm with the viral RNA genome into nucleocapsids (NCs). The two membrane proteins oligomerize soon after synthesis in the ER into p62E1 heterodimers and are then transported to the cell surface in order to take part in the budding process (Ziemiecki et al., 1980;Wahlberg et al., 1989). In the released virus particles the heterodimers are clustered into groups of three, each representing a spikelike projection on the virus surface (Vogel et al., 1986;Fuller, 1987). Furthermore, the original p62E1 heterodimer is processed by a limited proteolytic attack 66 residues from the NH2 terminus of p62 to form mature E2E1 complexes . This cleavage occurs at a very late stage during cell surface transport of the heterodimer and is probably mediated by a host protease (de Curtis and Simons, 1988) .1. Abbreviations used in this paper: HA, hemagglutinin; NC, nucleocapsid; pfu, plaque-forming units ; PM, plasma membrane ; SFV, Semliki Forest virus .

Structures of vesicular stomatitis virus glycoprotein: membrane fusion revisited

“…They lie flat or nearly flat at the viral surface and their fusion loops are buried at a dimer interface. Upon lowpH exposure, dimers dissociate and the protomers reassociate in a trimeric structure [46,47]. Similar to the structure of post-fusion class I proteins [48], the fusion loops and the transmembrane domains are then located at the same end of an elongated molecule that is now perpendicular to the membrane [49,50] ( Fig.…”

Molecular Dissection of the Semliki Forest Virus Homotrimer Reveals Two Functionally Distinct Regions of the Fusion Protein

“…The E1 homotrimer was originally discovered because it is highly resistant to trypsin digestion and because the trimer interaction is stable to gradient sedimentation in the presence of the nonionic detergent NP-40 and to mild sodium dodecyl sulfate (SDS) treatment followed by SDS-polyacrylamide gel electrophoresis (PAGE) (49,50). In vivo, formation of the homotrimer occurs during virus entry into cells, is blocked by agents that raise the pH of endosomes, and is coincident with virus fusion in the endosome (29,43,50). In vitro, homotrimer formation occurs with kinetics, pH dependence, and lipid preferences similar to those of fusion (4,5,16,20,26,43).…”

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“…20,23,24 E1 causes cell fusion, whereas E2 is predominantly the protein responsible for cell membrane attachment. 19,[25][26][27][28][29] Importantly, the Sindbis E1 protein can fuse to cells independently of the receptor binding E2 protein. 26 Dubuisson and Rice 30 showed that the receptor-binding properties of Sindbis virus could be disrupted by insertion of short peptides (11 aa) into defined regions of the E2 glycoprotein but with retention of all other replication functions.…”