Structural changes in HIV maturation Nascent HIV particles assemble at the plasma membrane of an infected cell and bud into a membrane-enveloped, immature virion. Assembly and budding are driven by a polyprotein called Gag, which consists of a matrix domain (MA) that is recruited to the plasma membrane, a capsid domain (CA) responsible for self-assembly, and a nucleocapsid domain (NC) that recruits the viral RNA genome. Gag cleavage results in a structural rearrangement that produces the mature virion. Qu et al . imaged mature and immature HIV particles by electron tomography and focused in on the MA domain (see the Perspective by Hikichi and Freed). They found that MA rearranges between two distinct hexameric lattices, and mature MA modulates the viral membrane by binding to a membrane lipid. This finding suggests that MA may play functional roles in the mature virion. —VV