Alternative processing of proproteins in Aspergilli kexB gene disruptants under hyperosmotic conditions

Disruption of the kexB gene encoding a subtilisin-like processing protease in Aspergillus oryzae and Aspergillus nidulans led to remarkable morphological defects, and these phenotypes were suppressed under hyperosmotic conditions. In this study, we investigated to determine whether non-KexB proteases might complement the in vivo function of KexB in the two Aspergillus kexB disruptants. Neither overexpression of opsA or opsB encoding A. oryzae aspartyl proteases homologous to yeast yapsins (YPS1/2) suppressed the kexB mutation, although yapsins are multicopy suppressors for the yeast kex2 mutation. A. nidulans and A. oryzae kexB disruptants grown under hyperosmotic conditions processed a recombinant fusion protein carrying a synthetic dibasic processing site (Lys-Arg) although the disruptants grown under normal growth conditions did not cleave the site. These results suggest that the two Aspergilli have other potential processing proteases that are induced and/or activated under hyperosmotic conditions and consequently complement, at least in part, the in vivo function of KexB.