Spectral analysis and tryptic susceptibility as probes of HIV-1 capsid protein structure

The structures of HIV-1 capsid protein (CA, p24) isolated from mature virions and CA protein autoprocessed from a recombinant Gag-Pol precursor expressed in Escherichia coli were compared using circular dichroic (CD) spectral analysis. The spectra obtained for the intact recombinant and viral proteins were indistinguishable, indicating that the backbone configurations directed by the primary amino acid sequences of the proteins were similar or identical. The structure predictions derived from CD were, in general, inconsistent with a model proposing the eight-stranded beta barrel motif found in several RNA viruses. However, aspects of the model were supported by experiments that identified surface-exposed regions. Biochemical analysis indicated that the recombinant CA protein formed nonrandom higher-ordered structures in vitro. Under physiological conditions, the protein assembled into oligomers containing subunits in two packing arrangements. In one arrangement, the central region near Arg100 was exposed and susceptible to tryptic digestion at low enzyme concentrations (enzyme:substrate ratios = 1:5000 to 1:100). Also, in this arrangement, the proteins were susceptible to crosslinking by the bifunctional agent DTSSP. Proteins in the other arrangement were resistant to proteolysis at low enzyme concentrations. The central region of these resistant molecules was inaccessible to monospecific antibodies that recognized antigenic sites between residues 94 and 107 and these proteins were not crosslinked by DTSSP or EGS. Following incubation with trypsin, both the resistant molecules and the fragments derived from the susceptible proteins in the oligomer migrated as smaller complexes, suggesting that the regions digested by trypsin stabilized the oligomer unit. The results indicate that the central region of the HIV-1 CA protein plays a role in formation of higher-ordered structures. Moreover, the relative stability of the N- and C-terminal partial digestion fragments arising from cleavage at Arg100/Gly101 suggests that this exposed central region separates two structural domains of the protein.