ASSEMBLY AND COMPOSITION OF INTRACELLULAR PARTICLES FORMED BY MOLONEYMURINE LEUKEMIA-VIRUS

Assembly of type C retroviruses such as Moloney murine leukemia virus (M-MuLV) ordinarily occurs at the plasma membranes of infected cells a nd absolutely requires the particle core precursor protein, Pr65gag. P reviously we have shown that Pr65gag is membrane associated and that a t least a portion of intracellular Pr65gag protein appears to be route d to the plasma membrane by a vesicular transport pathway. Here we sho w that intracellular particle formation can occur in M-MuLV-infected c ells. M-MuLV immature particles were observed by electron microscopy b udding into and within rough endoplasmic reticulum, Golgi, and vacuola r compartments. Biochemical fractionation studies indicated that intra cellular Pr65gag was present in nonionic detergent-resistant complexes of greater than 150S. Additionally, viral RNA and polymerase function s appeared to be associated with intracellular particles, as were Gag- beta-galactosidase fusion proteins which have the capacity to be incor porated into virions. Immature intracellular particles in postnuclear lysates could be proteolytically processed in vitro to mature forms, w hile extracellular immature M-MuLV particles remained immature as long as 10 h during incubations. The occurrence or M-MuLV-derived intracel lular particles demonstrates that Pr65gag can associate with intracell ular membranes and indicates that if a plasma membrane Pr65gag recepto r exists, it also can be found in other membrane compartments. These r esults support the hypothesis that intracellular particles may serve a s a virus reservior during in vivo infections.