AP BIOLOGY

Detailed explanation-1: -The envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1) consists of a complex of gp120 and gp41. gp120 determines viral tropism by binding to target-cell receptors, while gp41 mediates fusion between viral and cellular membranes.

Detailed explanation-2: -HIV target cells must permit the fusion of the virus membrane and cell membrane by the sequential interaction of the virus envelope glycoproteins (gp120) with cell surface CD4 and one of two chemokine (CC) receptor molecules, CXCR4 or CCR5.

Detailed explanation-3: -Interactions between the viral envelope glycoprotein gp120 and the cell surface receptor CD4 are responsible for the entry of human immunodeficiency virus type 1 (HIV-1) into host cells in the vast majority of cases.

Detailed explanation-4: -The precursor polyprotein of HIV-1 Env (gp160) is cleaved by a cellular protease into a surface unit (gp120) and a transmembrane unit (gp41), which are noncovalently bound to form a trimer on virions. HIV-1 gp120 and gp41 are responsible for virus binding to target cells and membrane fusion, respectively.