Opening Image: The HIV matrix protein, p17, forms the outer shell of the core of the virus, lining the inner surface of the viral membrane. The protein has several key functions. It orchestrates viral assembly via targeting signals that direct the gag precursor polyprotein, p55, to the host cell membrane. It also interacts with the transmembrane protein, gp41, to retain the env-encoded proteins in the virus. In addition, p17 contains a nuclear localization signal that directs the preintegration complex to the nucleus of infected cells. This permits the virus to infect nondividing cells, a distinguishing feature of HIV and other retroviruses.

p17 has a polar surface that interacts both with water and with other viral proteins. However, a number of hydrophobic residues are also on the outside. Toggle between CPK and relative temperature color schemes.

Toggle polar sidechains only (non-polar sidechains are shown as trace model).

p17 contains seven Arg residues. Where would you look for the positively charged guanidino group of Arg?

Highlight guanidino groups.

Tyr and Trp have hydrogen-bonding –OH and –NH groups, respectively.

p17 contains three tyrosines. The hydrophobic parts of Tyr86 and Tyr29 are buried on the inside. Only the –OH group of Tyr86 is exposed. Tyr79 lies in a hydrophobic cleft with one side exposed. Toggle spacefill.

p17 contains two tryptophans. In the crystal structure, Trp36 is completely exposed on one side, while Trp16 is completely buried in the structural core; its indole –NH group is hydrogen-bonded to a backbone carbonyl oxygen. Only one side of the large aromatic indole moiety of Trp36 lies against the protein, leaving the other hydrophobic surface exposed.

Toggle surface model. Tyr and Trp are shown in spacefill. Notice how the hydrogen donors project from the surface.