SIGNAL PEPTIDASE CLEAVAGE AT THE FLAVIVIRUS C-PRM JUNCTION - DEPENDENCE ON THE VIRAL NS2B-3 PROTEASE FOR EFFICIENT PROCESSING REQUIRES DETERMINANTS IN C, THE SIGNAL PEPTIDE, AND PRM

Signal peptidase cleavage at the C-prM junction in the flavivirus stru ctural polyprotein is inefficient in the absence of the cytoplasmic vi ral protease, which catalyzes cleavage at the COOH terminus of the C p rotein. The signal peptidase cleavage occurs efficiently in circumstan ces where the C protein is deleted or if the viral protease complex is present. In this study, we used cDNA of Murray Valley encephalitis vi rus (MVE) to examine features of the structural polyprotein which allo w this regulation of a luminal cleavage by a cytoplasmic protease. We found that the inefficiency of signal peptidase cleavage in the absenc e of the viral protease is not attributable solely to features of the C protein. Inhibition of cleavage still occurred when charged residues in C were mutated to uncharged residues or when an unrelated protein sequence (that of ubiquitin) was substituted for C. Also, fusion of th e C protein did not inhibit processing of an alternative adjacent sign al sequence. The cleavage region of the flavivirus prM translocation s ignal is unusually hydrophobic, and we established that altering this characteristic by making three point mutations near the signal peptida se cleavage site in MVE prM dramatically increased the extent of cleav age without requiring removal of the C protein. In addition, we demons trated that luminal sequences downstream from the signal peptidase cle avage site contributed to the inefficiency of cleavage.