| The capsid domain of retroviral Gag proteins possesses 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 will impair virus infectivity, the precise structure and function of the MHR is not known. In order to obtain further information on the MHR, the structures of synthetic peptides encompassing the MHRs of the human immunodeficiency virus type I (HIV-1) and Moloney murine leukemia virus (MoMLV) capsid proteins were determined by several techniques.; Multiple sequence alignment and secondary structure prediction indicated that the HIV-1 peptide could form 50% {dollar}alpha{dollar}-helix while the MoMLV peptide may be 38% {dollar}alpha{dollar}-helix. Additionally, circular dichroism studies indicated that, in the presence of 50% trifluoroethanol, the HIV-1 peptide adopts an {dollar}alpha{dollar}-helical structure over half of its length and the MoMLV peptide is over one third {dollar}alpha{dollar}-helix. Further analysis by proton nuclear magnetic resonance spectroscopy suggested that the C-terminal portion of the MHR of each virus forms a helix in aqueous solution. Upon the addition of trifluoroethanol, the position of the helix remains nearly constant, but the magnitude of the changes in H{dollar}sbalpha{dollar}, chemical shifts of the residues indicated a more stable helix. NMR data from nuclear Overhauser effect spectroscopy experiments was later used to generate molecular models of the peptides. The C-terminal half of both peptides formed an {dollar}alpha{dollar}-helix, while the N-terminal half was disordered.; Remarkable agreement was observed among the results of each technique for each peptide in terms of the placement and the size of the helix. The position of the helix was common to both peptides indicating a conservation of secondary structure. Furthermore, the observed helices were amphipathic, with one side occupied by hydrophobic amino acid side chains and the other side by hydrophilic side chains. These results suggest that a helical C-terminus of retroviral MHRs may be integral to the function of this region, possibly as a point of interaction between capsid protein monomers or between the capsid protein and other viral components. |
