Retroviruses contain a small protein, MA (matrix), which forms a protein lining immediately beneath the phospholipid membrane of the mature virus particle. MA is located in the N-terminal region of the Gag precursor polyprotein. The N-terminal segment of MA proteins directs the Gag protein to the plasma membrane where budding takes place, and has been called the M domain. This domain forms an alpha helical bundle structure.

1: Solution structure and dynamics of the bioactive retroviral M domain from Rous sarcoma virus. McDonnell JM, Fushman D, Cahill SM, Zhou W, Wolven A, Wilson CB, Nelle TD, Resh MD, Wills J, Cowburn D J Mol Biol 1998;279:921-928. PMID:9642071