| Description |
The human immunodeficiency virus type 1 (HIV-1) initially assembles as a spherical, immature particle organized by the Gag polyprotein. As the virus buds, Gag is cleaved into three structural proteins: MA, CA, and NC. Upon Gag processing, CA condenses to form the conical viral capsid of the mature virus. Proper assembly of the capsid is essential for viral infectivity, and this process requires accurate and ordered processing at both ends of CA. The structures of the N- and C-terminal domains of CA (NTD and CTD), which presumably represent CA in its mature conformation, were solved previously. To examine CA in its immature conformation, CA molecules with N- and C terminal extensions were characterized biochemically and structurally. The solution structure of the N-terminal domain of CA fused to the final four MA residues (129MA-CA278) revealed that there are significant structural rearrangements at the N-terminal end of CA upon proteolysis. In comparison with the structure of fully processed CA NTD, the N-terminal beta-hairpin and surrounding helices 1, 3, and 6 are significantly reoriented. Thus, we speculate that these rearrangements are required for the conformational polymorphism displayed by CA, either in the pentamer and hexamer formation of the mature viral capsid, or in viral maturation. The structure of the C-terminal domain of CA fused to the 14-residue p2 region (CA146-p2) was determined by X-ray crystallography. Unfortunately, the region encompassing the C-terminus of CTD and p2 residues was disordered in the crystal lattice and no conformational differences were observed in comparison of the CA146-p2 with the fully processed CA CTD. However, this result does not exclude the possibility that this region may adopt ordered structures when engaging in Gag-Gag interactions, which could help regulate capsid maturation. Inhibitors that target the structural CA protein are potentially complementary to the drugs used in current combination chemotherapy (inhibitors of HIV-1 protease and reverse transcriptase). Three assays that can be used to identify inhibitors of CA maturation, CA dimerization, and CA-NC assembly were therefore designed and optimized. Potentially, inhibiting different stages of capsid assembly will provide a novel mechanism of action for treating HIV/AIDS. |
