The Cellular Protein La Functions in Enhancement of Virus Release through Lipid Rafts Facilitated by Murine Leukemia Virus Glycosylated Gag

Abstract

ABSTRACT Murine leukemia viruses (MuLVs) encode two forms of Gag polyprotein: the precursor for the viral core proteins (Pr65gag for Moloney MuLV [M-MuLV]) and a longer glycosylated form (glyco-gag, or gPr80gag). gPr80gag is translated from the same unspliced viral RNA as Pr65gag, from an upstream in-frame CUG initiation codon. As a result, gPr80gag contains 88 unique N-terminal amino acids that include a signal peptide that conducts gPr80gag into the rough endoplasmic reticulum, where it is glycosylated, exported to the cell surface, and cleaved into two proteins of 55 and 40 kDa. The amino-terminal 55-kDa protein remains cell associated with the 88 unique amino acids exposed to the cytosol. We previously showed that gPr80gag facilitates efficient M-MuLV release through lipid rafts. In this report, we found that the unique N-terminal domain of gPr80gag is sufficient to facilitate enhanced M-MuLV particle release from transfected 293T cells. A search for cellular proteins involved in gPr80gag function led to cellular La protein. Overexpression of mouse or human La enhanced M-MuLV particle release in the absence of glyco-gag, and the released virus had a reduced buoyant density characteristic of increased cholesterol content. Moreover, small interfering RNA (siRNA) knockdown of human La abolished glyco-gag enhancement of M-MuLV release. These results implicate La as a cellular protein involved in M-MuLV glyco-gag function. We also found that overexpression of mouse or human La could enhance HIV-1 release in the absence of gPr80gag. Therefore, M-MuLV and HIV-1 may share a pathway for release through lipid rafts involving La. IMPORTANCE Retroviruses cause diseases such as leukemia and AIDS. An important aspect of viral replication is how viruses are released from infected cells. We previously found that a unique protein encoded by murine leukemia viruses (MuLVs), glyco-gag (or gPr80gag), enhances efficient virus release through cholesterol-rich membrane subdomains called lipid rafts. In this study, we found that the N-terminal domain of gPr80gag is sufficient to enhance viral release. A search for cellular proteins that participate in gPr80gag function led to cellular La protein. Overexpression of La phenocopied glyco-gag in enhancing M-MuLV release, and knockdown of La abolished glyco-gag function. M-MuLV glyco-gag also enhanced release of HIV-1, as did overexpression La in the absence of glyco-gag. Thus, M-MuLV and HIV-1 may share a cellular pathway for release through lipid rafts involving La. These results may also be relevant for other viruses that are released through lipid rafts.