Secretion of Saccharomyces cerevisiae killer toxin: processing of the glycosylated precursor

Killer toxin secretion was blocked at the restrictive temperature in Saccharomyces cerevisiae sec mutants with conditional defects in the S. cerevisiae secretory pathway leading to accumulation of endoplasmic reticulum (sec18), Golgi (sec7), or secretory vesicles (sec1). A 43,000-molecular-weight (43K) glycosylated protoxin was found by pulse-labeling in all sec mutants at the restrictive temperature. In sec18 the protoxin was stable after a chase; but in sec7 and sec1 the protoxin was unstable, and in sec1 11K toxin was detected in cell lysates. The chymotrypsin inhibitor tosyl-l-phenylalanyl chloromethyl ketone (TPCK) blocked toxin secretion in vivo in wild-type cells by inhibiting protoxin cleavage. The unstable protoxin in wild-type and in sec7 and sec1 cells at the restrictive temperature was stabilized by TPCK, suggesting that the protoxin cleavage was post-sec18 and was mediated by a TPCK-inhibitable protease. Protoxin glycosylation was inhibited by tunicamycin, and a 36K protoxin was detected in inhibited cells. This 36K protoxin was processed, but toxin secretion was reduced 10-fold. We examined two kex mutants defective in toxin secretion; both synthesized a 43K protoxin, which was stable in kex1 but unstable in kex2. Protoxin stability in kex1 kex2 double mutants indicated the order kex1 --> kex2 in the protoxin processing pathway. TPCK did not block protoxin instability in kex2 mutants. This suggested that the KEX1- and KEX2-dependent steps preceded the sec7 Golgi block. We attempted to localize the protoxin in S. cerevisiae cells. Use of an in vitro rabbit reticulocyte-dog pancreas microsomal membrane system indicated that protoxin synthesized in vitro could be inserted into and glycosylated by the microsomal membranes. This membrane-associated protoxin was protected from trypsin proteolysis. Pulse-chased cells or spheroplasts, with or without TPCK, failed to secrete protoxin. The protoxin may not be secreted into the lumen of the endoplasmic reticulum, but may remain membrane associated and may require endoproteolytic cleavage for toxin secretion.