HIV-1 envelope glycoprotein (Env) is a trimer of non-covalently linked heterodimer of gp120 and gp41 subunits. Gp120 is composed of five conserved regions (C1-5) that are interspersed with 5 variable regions (V1-5). Despite less conserved nature of the sequences, variable loops may be functionally important for virus entry and constitute neutralization determinants. To investigate the importance of loops for Env biology, we generated a series of loop deletion mutants and investigated the effect of each loop deletion on Env structural integrity, and on Env expression, cell surface display, virus packaging and entry into permissive cells. We found that loop V4 and V5 were important for envelope structural integrity. Replacement of V4 or V5 with a flexible linker led to loss of the binding of CD4 binding site antibodies and CD4-induced antibodies to the Env, but enhanced the exposure of the membrane-proximal external region. Although replacement of V4 or V5 with the flexible linker did not affect Env expression in cells, it significantly affected Env display on cell surface, leading to the failure in virus packaging and subsequent entry into permissive cells. HIV-1 Env variable loop V4 and V5 are indispensable for Env structural integrity. Deletion of V4 and V5 significantly affected Env cell surface display, virus packaging and subsequent virus entry into the cells.
- Copyright © 2013 by The American Association of Immunologists, Inc.