EXPRESSION OF THE SACCHAROMYCES-CEREVISIAE KEX2P ENDOPROTEASE IN INSECT CELLS - EVIDENCE FOR A CARBOXY-TERMINAL AUTOPROCESSING EVENT

The pheromone-processing Kex2p endoprotease of Saccharomyces cerevisiae has been difficult to characterize due to its low level of expression in yeast cells. To overcome this problem, we have overexpressed Kex2p using the baculovirus/insect cell expression system. Spodoptera frugiperda Sf9 insect cells infected with a recombinant baculovirus, containing the complete KEX2 gene which encodes the Kex2p protease (814 amino acids), accumulate an 120-kDa functional form of the enzyme. The inhibition profile of the insect-cell-derived endoprotease is similar to that of the yeast enzyme. The recombinant infected insect cells also secrete into the medium about half of the total Kex2p activity produced. Deleting the carboxyl-terminal tail and the transmembrane domain of Kex2p (Kex2-DELTA-p, 666 amino acids) does not measurably interfere with the enzyme characteristics and results in the secretion of up to 90% of the total enzyme activity. The truncated form, Kex2-DELTA-p, of the endoprotease accumulates in the cell supernatant to 6.7 x 10(5) U/1. The molecular mass of the secreted forms for both the wild-type Kex2p and Kex2-DELTA-p is the same (70 kDa) and is 50-kDa lower than the intracellular form. This result implicates a processing event which gives rise to shorter extracellular forms of both the wild-type Kex2p and Kex2-DELTA-p and which trims their carboxy termini upstream of amino acid 666. This processing event requires the integrity of the Ser385 of the Kex2p active site.