PLASMA-MEMBRANE TARGETING OF CHIMERIC INTRACISTERNAL A-TYPE PARTICLE POLYPROTEINS LEADS TO PARTICLE RELEASE AND SPECIFIC ACTIVATION OF THE VIRAL PROTEINASE

Retrovirus morphogenesis involves assembly of structural Gag polyprote ins with subsequent budding from the plasma membrane, followed by prot eolytic cleavage by the viral proteinase (PR) and extracellular matura tion to the infectious virion, Intracisternal A-type particles (IAPs) are defective retroviruses that assemble and bud at the membranes of t he endoplasmic reticulum (ER), where they remain as immature particles consisting exclusively of uncleaved polyproteins, To analyze requirem ents for intracellular polyprotein transport and PR activation, we con structed deletion and substitution mutations in the IAP gag gene, incl uding the putative ER-targeting signal, Mutant polyproteins were trans ported to various intracellular locations, including the nucleus, the cytoplasm, the ER, and the plasma membrane, Interestingly, assembly of capsid-like particle structures occurred at almost all sites, However , only those polyproteins transported to the plasma membrane were effi ciently and specifically cleaved by viral PR, with cleavage occurring predominantly within the virus particle. Thus, at least in the experim ental system presented here, retroviral particle assembly can occur at almost any location within the cell, while polyprotein processing and , consequently, virion maturation are confined to a specific cellular site, These results suggest that a factor restricted to the plasma mem brane is required to trigger PR activation and maturation of infectiou s retroviruses.