THE ROLE OF CUTICLE-DEGRADING PROTEASES IN FUNGAL PATHOGENESIS OF INSECTS

The proteinaceous outer integument of insects forms an effective barrier against most microbes. Only the 700 known species of entomopathogenic fungi effect entry into their hosts by breaching the cuticle. There is accumulating evidence that the ability of fungi to degrade protein may aid their invasion of and growth in this orderly complex structure. Evidence for the particular importance of proteinases derives largely from studies of their production in infected cuticles associated with cuticle degradation, the effects of proteinase inhibitors on pathogen behavior, and by the analysis of protease-deficient mutants. More recently, studies have included the cloning, identification, and manipulation of specific protease genes of Metarhizium anisopliae, particularly those of the subtilisin (chymoelastase) type (designated Prl) also produced by many other entomopathogenic fungi. Following solubilization of cuticle proteins by Prl-type endoproteases, complete degradation of the cuticle involves a number of interacting enzyme species including a family of trypsin-like proteinases, metalloproteinases, several aminopeptidases, and a carboxypeptidase. Testing genetically engineered M. anisopliae null mutants of Prl indicated that the other endopeptidases can partially substitute for Prl. The exopeptidases further degrade peptides released by the endopeptidases producing free amino acids for uptake and metabolism. Utilization of these enzymes has assisted investigators in understanding cuticle structure and how the cuticle is degraded naturally, and could lead to improved strain selection of entomopathogenic fungi or the introduction of their genes into other microbes and plants for the purpose of insect control.