A CRITICAL PROTEOLYTIC CLEAVAGE SITE NEAR THE C-TERMINUS OF THE YEASTRETROTRANSPOSON TY1 GAG PROTEIN

Cleavage of the Gag and Gag-Pol polyprotein precursors is a critical s tep in proliferation of retroviruses and retroelements. The Ty1 retroe lement of Saccharomyces cerevisiae forms virus-like particles (VLPs) m ade of the Gag protein. Ty1 Gag is not obviously homologous to the Gag proteins of retroviruses. The apparent molecular mass of Gag is reduc ed from 58 to 54 kDa during particle maturation. Antibodies raised aga inst the C-terminal peptide of Gag react with the 58-kDa polypeptide b ut not with the 54-kDa one, indicating that Gag is proteolytically pro cessed at the C terminus. A protease cleavage site between positions 4 01 and 402 of the Gag precursor was defined by carboxy-terminal sequen cing of the processed form of Gag. Certain deletion and substitution m utations in the C terminus of the Gag precursor result in particles th at are two-thirds the diameter of the wild-type VLPs, While the Ty1 pr otease is active in these mutants, their transposition rates are decre ased 20 fold compared with that of wild-type Ty1. Thus, the Gag C-term inal portion, released in the course of particle maturation, probably plays a significant role in VLP morphogenesis and Ty1 transposition.