C H A P T E R 3 5 , F IG U R E 16

CD4 (cluster differentiation 4) protein. CD4 on T cells is a component of the

MHC class II, TcR complex. Shown are the extracellular domains that consist of two immunologlobulin-like folds.

The (5-sheet secondary structure is illustrated in one fold; only the a-carbon backbone is shown in the other. The fig-

ure is derived from the coordinates published in the Protein Data Bank file 1WIO.

C H A P T E R 3 5 , F IG U R E 17

Complex of HIV Gpl20, Fab fragment, and CD4. HIV gpl20 is an envelope pro-

tein of HIV-1 that is exposed on the outer surface of the virus. It is derived from gpl60 by proteolysis that also pro-

duces another product, gp41. Only a small area of the surface of the gpl20 and CD4 molecules is in contact.

Interaction between CD4 and gpl20 is dominated by residues Phe43 and Arg59 of CD4, and Gly370, Try427, and

Asp368 of Gpl20. The antibody fragment blocks the region of Gpl20 that interacts with a chemokine receptor,

CCR5. The figure is derived from the coordinates published in the Protein Data Bank file 1GC1.