The matrix protein is at the N terminus of the GAG polyprotein. It lines the inner surface of the viral membrane (or the infected cells before bidding). At the N terminus is a site to which myristic acid is attached and this allows the gag/pol to associated with the lipid bilayer of the membrane. Also there are basic residues in the first 50 amino acids and three molecules associate to form a trimer or triskelion-like structure. If the trimer cannot form as a result of mutation, the virus cannot assemble since it appears that lysines that are important for attachment to the membrane form a membrane-binding surface on the trimer. The trimer has three myristate molecules which embed in the membrane while the lysine-containing region associated with the negatively charged phospholipid head groups.MA is also instrumental in the incorporation of the cytoplasmic end of the TM protein into the viral particle and participates in the infection of non-dividing cells (e.g. macrophages) by an unknown mechanism. MA has been reported to contain a nuclear localization signal and may be part of the pre-integration complex

For further information see: Alan Frankel and John A. T. Young, HIV-1: Fifteen proteins and an RNA Annual Review of Biochemistry 67: 1-25, 1998