Final answer:
a. GP120 and GP41.
HIV's entry into a target cell is facilitated by the interaction of its membrane glycoproteins GP120 and GP41 with the CD4 receptor on T cells, leading to fusion and infection.
Step-by-step explanation:
The binding and fusion of HIV into a target cell occurs through the interaction of two membrane glycoproteins: GP120 and GP41. GP120 first binds to the CD4 receptor, a glycoprotein located on the surfaces of T cells.
This binding is crucial as it allows the viral envelope to fuse with the cell membrane through the action of GP41. Once fusion occurs, HIV can release its contents into the target cell, initiating the process of infection.