gp41

gp41 is a glycoprotein non-covalently bound to gp120, and provides the second step by which HIV enters the cell. It is originally buried within the viral envelope, but when gp120 binds to a CD4 receptor, gp120 changes its conformation causing gp41 to become exposed, where it can assist in fusion with the host cell.
See more at Wikipedia.org...

protein on the outer surface of HIV that can pierce the surface of a helper T-lymphocyte cell (t-cell) This is HIV's infection mechanism.

Glycoprotein 41. One of the proteins embedded in the outer envelope of HIV. gp41 plays a key role in HIV's infection of CD4 cells by fusing HIV's envelope with the host cell membrane, allowing the virus to enter the cell.
See Also: Envelope, Fusion Inhibitors
  

(gp41) Glycoprotein 41, a protein embedded in the outer envelope<!-- (see) --> of HIV. Plays a key role in HIV's infection of CD4+ T cells<!-- (see) --> by facilitating the fusion of the viral and the cell membranes. See gp120.

Glucoproteína 41. Una de las proteínas dentro de la envoltura externa del VIH. Desempeña un papel muy importante en la infección de los linfocitos CD4 por el VIH al fusionar la envoltura de ese virus con la membrana de la célula anfitriona, permitiéndole al virus penetrar en la célula.
Véase también: Envoltura, Inhibidores de la fusión