The human immunodeficiency virus type I (HIV-1) capsid protein is initially
synthesized as the central domain of the Gag polyprotein, and is subsequen
tly proteolytically processed into a discrete 231-amino-acid protein that f
orms the distinctive conical core of the mature virus. The crystal structur
es of two proteins that span the C-terminal domain of the capsid are report
ed here: one encompassing residues 146-231 (CA(146-231)) and the other exte
nding to include the 14-residue p2 domain of Gag (CA(146-p2)). The isomorph
ous CA(146-231) and CA(146-p2) structures were determined by molecular repl
acement and have been refined at 2.6 Angstrom resolution to R factors of 22
.3 and 20.7% (Re-free = 28.1 and 27.5%), respectively. The ordered domains
comprise residues 148-219 for CA(146-231) and 14-218 for CA(146-p2), and th
eir refined structures are essentially identical. The proteins are composed
of a 3(10) helix followed by an extended strand and four alpha-helices. A
crystallographic twofold generates a dimer that is stabilized by parallel p
acking of an alpha-helix 2 across the dimer interface and by packing of the
310 helix into a groove created by alpha-helices 2 and 3 of the partner mo
lecule. CA(146-231) and CA(146-p2) dimerize with the full affinity of the i
ntact capsid protein, and their structures therefore reveal the essential d
imer interface of the HIV-1 capsid.