Palmitoylation of the Autographa californica Multicapsid Nucleopolyhedrovirus Envelope Glycoprotein GP64: Mapping, Functional Studies, and Lipid Rafts

Abstract

Budded virions (BV) of the baculovirus Autographa californica multicapsid nucleopolyhedrovirus (Ac M NPV) contain a major envelope glycoprotein known as GP64, which was previously shown to be palmitoylated. In the present study, we used truncation and amino acid substitution mutations to map the palmitoylation site to cysteine residue 503. Palmitoylation of GP64 was not detected when Cys503 was replaced with alanine or serine. Palmitoylation-minus forms of GP64 were used to replace wild-type GP64 in Ac M NPV, and these viruses were used to examine potential functions of GP64 palmitoylation in the context of the infection cycle. Analysis by immunoprecipitation and cell surface studies revealed that palmitoylation of GP64 did not affect GP64 synthesis or its transport to the cell surface in Sf9 cells. GP64 proteins lacking palmitoylation also mediated low-pH-triggered membrane fusion in a manner indistinguishable from that of wild-type GP64. Cells infected with viruses expressing palmitoylation-minus forms of GP64 produced infectious virions at levels similar to those from cells infected with wild-type Ac M NPV. In combination, these data suggest that virus entry and exit in Sf9 cells were not significantly affected by GP64 palmitoylation. To determine whether GP64 palmitoylation affected the association of GP64 with membrane microdomains, the potential association of GP64 with lipid raft microdomains was examined. These experiments showed that: (i) Ac M NPV-infected Sf9 cell membranes contain lipid raft microdomains, (ii) GP64 association with lipid rafts was not detected in infected Sf9 cells, and (iii) GP64 palmitoylation did not affect the apparent exclusion of GP64 from lipid raft microdomains.