Basic residues in the Mason-Pfizer monkey virus gag matrix domain regulate intracellular trafficking and capsid-membrane interactions.

Mason-Pfizer monkey virus (M-PMV) capsids that have assembled in the cytoplasm must be transported to and associate with the plasma membrane prior to being enveloped by a lipid bilayer during viral release. Structural studies have identified a positive-charge density on the membrane-proximal surface of the matrix (MA) protein component of the Gag polyprotein. To investigate if basic amino acids in MA play a role in intracellular transport and capsid-membrane interactions, mutants were constructed in which lysine and arginine residues (R10, K16, K20, R22, K25, K27, K33, and K39) potentially exposed on the capsid surface were replaced singly and in pairs by alanine. A majority of the charge substitution mutants were released less efficiently than the wild type. Electron microscopy of mutant Gag-expressing cells revealed four distinct phenotypes: K16A and K20A immature capsids accumulated on and budded into intracellular vesicles; R10A, K27A, and R22A capsid transport was arrested at the cellular cortical actin network, while K25A immature capsids were dispersed throughout the cytoplasm and appeared to be defective at an earlier stage of intracellular transport; and the remaining mutant (K33A and K39A) capsids accumulated at the inner surface of the plasma membrane. All mutants that released virions exhibited near-wild-type infectivity in a single-round assay. Thus, basic amino acids in the M-PMV MA define both cellular location and efficiency of virus release.