SOLUTION STRUCTURES OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 (HIV-1) AND MOLONEY MURINE LEUKEMIA-VIRUS (MOMLV) CAPSID PROTEIN MAJOR-HOMOLOGY-REGION PEPTIDE ANALOGS BY NMR-SPECTROSCOPY

The capsid domain of retroviral Gag proteins possesses a single highly conserved subdomain termed the major homology region (MHR). While the mutagenesis of residues in the MHR will impair virus infectivity, the precise solution structure and function of the MHR is not known. To a id the structure/function characterization of the MHR, the structures of synthetic peptides encompassing the MHR of the human immunodeficien cy virus type I (HIV-1) and Moloney murine leukemia virus (MoMLV) caps id proteins were investigated by several techniques. Homology-based se condary-structure prediction suggested that the HIV-1 and MoMLV peptid es could form 50% and 38% alpha-helix, respectively. CD studies indica ted that, in the presence of 50% trifluoroethanol, the HIV-1 peptide a dopts an alpha-helical structure over half of its length, while the Mo MLV peptide is over one third alpha-helix. Further analysis by H-1-NMR suggested that the C-terminal portion of the MHR of each virus forms a helix in aqueous solution. Distance-geometry structures of each pept ide were calculated from NOE distance restraints and were refined by r estrained molecular dynamics. The C-terminal halves of both peptides w ere observed to be in an alpha-helical conformation, while the N-termi nal halves were disordered. Furthermore, both helices were amphipathic with high conservation of amino acid side-chain character, suggesting that a conserved helical MHR C-terminus is essential to retroviral ca psid protein function.