Targeted assembly and cytoplasmic transport of Mason-Pfizer monkey virus capsids

Simple retroviruses assemble the products of three genes, gag, pol, and env, together with the RNA genome, to form infectious virions. This concerted effort of assembly is achieved by targeting of the virus-encoded molecules to specific locations in the cell to provide both a physical and temporal opportunity for interactions. Mason-Pfizer Monkey Virus (M-PMV), the prototypic D-Type retrovirus, assembles Gag-containing precursor proteins around an RNA genome in the cytoplasm of cells to form capsids that are transported to the plasma membrane and enveloped as they bud into the extracellular environment. The lipid envelope that surrounds the virus contains the membrane-spanning Env glycoprotein complex that will mediate entry into the next cell. The distinct separation of the processes of M-PMV Gag assembly and budding make the virus an ideal system in which to study the processes of intracellular targeting and assembly of retroviral proteins. The results described in this dissertation demonstrate that the cytoplasmic targeting/retention signal (CTRS) located on the Gag polyprotein targets assembly of these molecules to a centriolar region of the cell. Likewise, Env is targeted to this region, presumably via its tyrosine-based endocytosis motif located on the cytoplasmic tail of Env.; The CTRS mediates transport of Gag polyproteins toward the minus-end of microtubules through interactions with the dynein/dynactin molecular motor complex and evidence is presented that the CTRS functions in a cotranslational manner. Following completion of assembly, the spherical capsids await a signal for export from the region that is provided by Env. This interaction between the integral membrane glycoprotein and cytoplasmic capsids occurs at the level of the recycling endosome, and Env trafficking through this compartment is necessary for efficient intracellular transport of assembled capsids.; Env-mediated tracking of Gag via endocytic membranes elucidates the manner in which viruses take advantage of their host by presenting trafficking signals in trans. The results of the studies described in this thesis from a virus that assembles its protein shell in the cytoplasm have direct relevance to the intracellular trafficking of Gag and Env molecules from pathogenic human retroviruses, such as human immunodeficiency virus or human T-cell leukemia virus.