Figure 9: Three-dimensional structures of MA, CA, and NC. (A and B courtesy of Wes Sundquist; C courtesy of Mike Summers.) (A) HIV MA monomer. The drawing shows the polypeptide backbone as observed in the crystal structure of MA purified after expression in E. coli, without amino-terminal myristylation. (B) HIV MA trimer. Hypothetical juxtaposition of HIV MA and viral membrane. The drawing shows the MA trimer as observed in the crystal structure positioned next to a sheet of lipid as might be found in the viral envelope. (Yellow) Lipid head groups; (orange) myristyl group from each monomer inserted into the membrane; (purple) positively charged side chains known to be involved in membrane interaction; (green) positively charged side chains not involved in membrane interaction. (C) HIV NC monomer. The drawing shows the polypeptide backbone as inferred from NMR studies, with the two Cys-His motifs each binding a Zn++ ion. The amino-terminal and carboxy-terminal portions of the polypeptide projecting away from the Cys-His motifs are shown in an arbitrary orientation, since they are not observed to be structured.