Cellular Receptors and Viral Glycoproteins Involved in Retrovirus Entry

Abstract

The initial steps in virus infection involve the binding of virions to specific cell surface receptors. Following the attachment of enveloped viruses, further events lead to membrane fusion either at the cell surface or in endocytic vesicles that deliver the core of the virus to the cytoplasm (Marsh and Helenius, 1989). For retroviruses, the outer surface (SU) envelope glycoproteins bear epitopes for specific receptor recognition. Binding and conformational changes in the envelope spikes lead to the exposure of hydrophobic domains on the transmembrane (TM) envelope protein that are believed to effect membrane fusion by embedding in the plasma membrane. The individual envelope (env) proteins (p) and glycoproteins (gp) are named after their estimated molecular weight. Thus for the human immunodeficiency virus type 1 (HIV-1) the SU protein, gp120, is approximately 120 kDa, and the TM protein, gp41, is 41 kDa. The retroviral TM/SU spikes are trimers or tetramers (Einfeld and Hunter, 1988; Schawaller et al., 1989; Earl et al., 1990, 1992; C. D. Weiss et al., 1990). With HIV-1, a tetrameric complex may assemble via dimerization of TM dimers (Pinter et al., 1989; Rey et al., 1990).