Fig. 2. HIV-1 Gag undergoes major conformational changes during virus particle assembly.

HIV-1 Gag protein assumes compact conformations in solution, with both ends near each other in 3-dimensional space (i ) [22]. Both the N-terminal, MA domain (green) and the C-terminal, NC domain (red) are positively charged, but the MA domain (blue) has a much higher affinity for negatively charged phospholipids than for RNA; the NC domain exhibits the converse pattern. Nonspecific electrostatic interactions maintain the compact conformations of the protein when it is bound to membrane with negatively charged phospholipids (ii), or when bound to RNA (iv). However, when both types of ligands are present, the protein extends to the rod-shaped conformation present in authentic virus particles (iii) (see Box 1)[24].