SPECIFIC BINDING OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 GAG POLYPROTEIN AND NUCLEOCAPSID PROTEIN TO VIRAL RNAS DETECTED BY RNA MOBILITY SHIFT ASSAYS

Packaging of retroviral genomic RNA during virion assembly is thought to be mediated by specific interactions between the gag polyprotein an d RNA sequences (often termed the PSI or E region) near the 5' end of the genome. For many retroviruses, including human immunodeficiency vi rus type 1 (HIV-1), the portions of the gag protein and the RNA that a re required for this interaction remain poorly defined. We have used a n RNA gel mobility shift assay to measure the in vitro binding of puri fied glutathione S-transferase-HIV-1 gag fusion proteins to RNA ribopr obes. Both the complete gag polyprotein and the nucleocapsid (NC) prot ein alone were found to bind specifically to an HIV-1 riboprobe. Eithe r Cys-His box of NC could be removed without eliminating specific bind ing to the PSI riboprobe, but portions of gag containing only the MA a nd CA proteins without NC did not bind to RNA. There were at least two binding sites in HIV-1 genomic RNA that bound to the gag polyprotein: one entirely 5' to gag and one entirely within gag. The HIV-1 NC prot ein bound to riboprobes containing other retroviral PSI sequences almo st as well as to the HIV-1 PSI riboprobe.