Type D retrovirus gag polyprotein interacts with the cytosolic chaperonin TRiC

The carboxy terminus-encoding portion of the gag gene of Mason-Pfizer monke y virus (M-PMV), the prototype immunosuppressive primate type D retrovirus, encodes a 36-amino-acid, proline-rich protein domain that, in the mature v irion, becomes the p4 capsid protein. The p4 domain has no known role in M- PMV replication. We found that two mutants,vith premature termination codon s that remove half or all of the p4 domain produced lower levels of stable Gag protein and of self-assembled capsids. Interestingly, yeast two-hybrid screening revealed that p4 specifically interacted with TCP-1 gamma, a subu nit of the chaperonin TRiC (TCP-I ring complex). TRiC is a cytosolic chaper onin that is known to be involved in both folding and subunit assembly of a variety of cellular proteins. TCP-1 gamma also associated with high specif icity with the M-PMV pp24/16-p12 domain and human immunodeficiency virus p6 . Moreover, in cells, Gag polyprotein associated with the TRiC chaperonin c omplex and this association depended on ATP hydrolysis. In the p4 truncatio n mutants, the Gag-TRiC association was significantly reduced. These result s strongly suggest that cytosolic chaperonin TRiC is involved in Gag foldin g and/or capsid assembly. We propose that TRiC associates transiently with nascent M-PMV Gag molecules to assist in their folding. Consequently, prope rly folded Gag molecules carry out the intermolecular interactions involved in self-assembly of the immature capsid.