ANALYSIS OF THE ASSEMBLY FUNCTION OF THE HUMAN-IMMUNODEFICIENCY-VIRUSTYPE-1 GAG PROTEIN NUCLEOCAPSID DOMAIN

Previous studies have shown that in addition to its function in specif ic RNA encapsidation, the human immunodeficiency virus type 1 (HIV-1) nucleocapsid (NC) is required for efficient virus particle assembly. H owever, the mechanism by which NC facilitates the assembly process is not clearly established. Formally, NC could art by constraining the Pr 55(gag) polyprotein into an assembly-competent conformation or by mask ing residues which block the assembly process, Alternatively, the capa city of NC to bind RNA or make interprotein contacts might affect part icle assembly, To examine its role in the assembly process, we replace d the NC domain in Pr55(gag) with polypeptide domains of known functio n, and the chimeric proteins were analyzed for their abilities to dire ct the release of virus-like particles. Our results indicate that KC d oes not mask inhibitory domains and does not act passively, by simply providing a stable folded monomeric structure. However, replacement of NC by polypeptides which form interprotein contacts permitted efficie nt virus particle assembly and release, even when RNA was not detected in the particles. These results suggest that formation of interprotei n contacts by NC is essential to the normal HIV-1 assembly process.