TYPE-D RETROVIRUS CAPSID ASSEMBLY AND RELEASE ARE ACTIVE EVENTS REQUIRING ATP

Mason-Pfizer monkey virus (M-PMV), the prototype type D retrovirus, di ffers from most other retroviruses by assembling its Gag polyproteins into procapsids in the cytoplasm of infected cells, Once assembled, th e procapsids migrate to the plasma membrane,,where they acquire their envelope during budding, Because the processes of M-PMV protein transp ort, procapsid assembly, and budding are temporally and spatially unli nked, we have been able to determine whether cellular proteins play an active role during the different stages of procapsid morphogenesis. W e report here that at least two stages of morphogenesis require ATP. B oth procapsid assembly and procapsid transport to the plasma membrane were reversibly blocked by treating infected cells with sodium azide a nd 2-deoxy-D-glucose, which we show rapidly and reversibly depletes ce llular ATP pools. Assembly of procapsids in vitro in a cell-free trans lation/assembly system was inhibited by the addition of nonhydrolyzabl e ATP analogs, suggesting that STP hydrolysis and not just ATP binding is required, Since retrovirus Gag polyproteins do not bind or hydroly ze ATP, these results demonstrate that cellular components must play a n active role during retrovirus morphogenesis.