The N-terminal matrix domain (MA) of the HIV-1 Gag polyprotein targets and recruits Gag to the plasma membrane of virus producing cells. After virus budding, Gag is cleaved by the viral protease into its individual domains, triggering the morphological transformation of the non-infectious immature virus into its infectious mature form. We have shown recently that upon maturation MA rearranges its lattice and recruits a co-factor into a side binding pocket in the MA domain. However, the relatively low resolution of the reconstructions prevented a precise description of MA-MA interactions and of the binding pocket. Here we present high-resolution in situ structures of the wild-type mature MA lattice and of an immature-like MA lattice formed by a MA point mutant. These structures provide novel insights into how the MA domain binds and clusters lipids during Gag assembly and maturation. We also present experimental data towards determining how MA maturation is controlled and triggered.