Spectroscopic study of an HIV-1 capsid protein major homology region peptide analog

The capsid (CA) domain of retroviral Gag proteins posseses one subdomain, the major homology region (MHR), which is conserved among nearly all avian and mammalian retroviruses. While it is known that the mutagenesis of residues in the MHR mill impair virus infectivity, the precise structure and function of the MHR is not known. In order to obtain further information on the MHR, we have examined the structure of a synthetic peptide encompassing the MHR of human immunodeficiency virus type I (HIV-1) CA protein. Multiple sequence alignment and secondary structure prediction indicate that the peptide could form 50% alpha-helix and 10% beta-sheet. In addition, circular dichroism studies indicate that, in the presence of 50% trifluoroethanol (TFE), the peptide adopts an alpha-helical structure over half of its length. Further analysis by proton nuclear magnetic resonance spectroscopy suggests that the C-terminal portion of the MHR forms a helix in aqueous solution. Upon the addition of TFE, the position of the helix remains nearly constant, but the magnitude of the changes in H-alpha chemical shifts of the residues indicate a more stable helix. These results suggest that a helical C-terminus of retroviral MHRs may be integral to the function of this region.