A Particle-Associated Glycoprotein Signal Peptide Essential for Virus Maturation and Infectivity

Abstract

Signal peptides (SP) are key determinants for targeting glycoproteins to the secretory pathway. Here we describe the involvement in particle maturation as an additional function of a viral glycoprotein SP. The SP of foamy virus (FV) envelope glycoprotein is predicted to be unusually long. Using an SP-specific antiserum, we demonstrate that its proteolytic removal occurs posttranslationally by a cellular protease and that the major N-terminal cleavage product, gp18, is found in purified viral particles. Analysis of mutants in proposed signal peptidase cleavage positions and N-glycosylation sites revealed an SP about 148 amino acids (aa) in length. FV particle release from infected cells requires the presence of cognate envelope protein and cleavage of its SP sequence. An N-terminal 15-aa SP domain with two conserved tryptophan residues was found to be essential for the egress of FV particles. While the SP N terminus was found to mediate the specificity of FV Env to interact with FV capsids, it was dispensable for Env targeting to the secretory pathway and FV envelopemediated infectivity of murine leukemia virus pseudotypes.Signal peptides (SP) are key determinants for targeting and membrane insertion of secretory and membrane proteins (reviewed in reference 25). They can be removed co-or posttranslationally by the cellular membrane-bound signal peptidase or may, if not cleaved, serve as membrane anchors for proteins with distinct membrane orientations. In general, SP are composed of three domains, of which a central 6-to 15-amino-acid (aa)-long hydrophobic domain (h-domain) is the most essential. An N-terminal polar domain (n-domain) usually of net positive charge shows high variability in overall length, ranging from 15 to more than 50 aa. The composition and structure of the n-domain influences protein orientation in the membrane. The polar C-terminal domain (c-domain) often contains helixbreaking as well as small uncharged residues in positions -3 and -1 which determine the site of SP cleavage. In most cases, SP cleavage is thought to occur cotranslationally; however, for some proteins, e.g., the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp160, SP cleavage occurs inefficiently and very late after translocation (21). A basic amino acid stretch in the n-domain of gp160 is responsible for this phenomenon and believed to influence folding and exit of HIV-1 Env from the endoplasmic reticulum (ER) (21). Recent studies revealed that SP bear specific information accounting for distinct functions in targeting and membrane insertion or even for defined metabolic pathways after their cleavage from the parent protein (reviewed in reference 25). The HIV-1 SP Env , for example, is further processed by the signal peptidase, leading to the release of an SP fragment into the cytosol, where it binds to calmodulin (26). The function of this process in viral replication is not known.Foamy viruses (FV), as studied with the prototype member human foamy virus (HFV), follow a replication cycle which is charact...

Long-lived Signal Peptide of Lymphocytic Choriomeningitis Virus Glycoprotein pGP-C

“…A function in virus budding and integration into virus particles has also been shown for the SP of foamy virus Env protein (40). However, this SP does not share many structural similarities with the SP GP-C .…”

“…An unusually long and stable SP has also been found for the foamy virus envelope glycoprotein (Env) (40). The foamy virus SP Env is 148 amino acid residues in length and contains a single hydrophobic region located between residues 70 and 90.…”

Construction and characterization of efficient, stable and safe replication-deficient foamy virus vectors

“…These results show that type-specific interactions between N-terminal Env leader protein sequences and Gag are required as recently proposed although other factors may be also required. 29,30 We also tested whether FFV particles can be pseudotyped with the vesicular stomatitis virus glycoprotein VSV-G. 31 In repeated experiments, no marker gene transfer using VSV-G-pseudotyped FFV vectors was detectable confirming previous studies using simian/ primate FVs. 24 …”

“…Even the Env protein from the human/primate FV did not allow efficient marker gene transduction, underlining the concept that highly specific Gag-Env interactions are required for proper particle assembly although other reasons cannot be formally excluded. 29,30 In addition, we did not find a significant effect of Bet on the vector titer, as the 293T cells used for vector production are APOBEC3-deficient and thus, the deaminase-counteracting function of Bet is not required. 22 Additional effects of Bet on FFV vector particle release were not observed in this study.…”

Foamy virus–adenovirus hybrid vectors

“…The expression of Pol and Env proteins is shown in Figure 3b. The gp130 Env precursor and the gp18 Env leader peptide (LP) were detected in HC-FAD-2-transduced cells, 23 and the full-length Pol proteins in cells transduced with HC-FAD-2 and -7. In HC-FAD-8-transduced cells, a minor band migrating below the p127 Pol precursor was found.…”