Total chemical synthesis of N-myristoylated HIV-1 matrix protein p17: structural and mechanistic implications of p17 myristoylation.

The HIV-1 matrix protein p17, excised proteolytically from the N terminus of the Gag polyprotein, forms a protective shell attached to the inner surface of the plasma membrane of the virus. During the late stages of the HIV-1 replication cycle, the N-terminally myristoylated p17 domain targets the Gag polyprotein to the host-cell membrane for particle assembly. In the early stages of HIV-1 replication, however, some p17 molecules dissociate from the viral membrane to direct the preintegration complex to the host-cell nucleus. These two opposing targeting functions of p17 require that the protein be capable of reversible membrane interaction. It is postulated that a significant structural change in p17 triggered by proteolytic cleavage of the Gag polyprotein sequesters the N-terminal myristoyl group, resulting in a weaker membrane binding by the matrix protein than the Gag precursor. To test this "myristoyl switch" hypothesis, we obtained highly purified synthetic HIV-1 p17 of 131 amino acid residues and its N-myristoylated form in large quantity. Both forms of p17 were characterized by circular dichroism spectroscopy, protein chemical denaturation, and analytical centrifugal sedimentation. Our results indicate that although N-myristoylation causes no spectroscopically discernible conformational change in p17, it stabilizes the protein by 1 kcal/mol and promotes protein trimerization in solution. These findings support the premise that the myristoyl switch in p17 is triggered not by a structural change associated with proteolysis, but rather by the destabilization of oligomeric structures of membrane-bound p17 in the absence of downstream Gag subdomains.