Activation of the Yeast Cell Wall Integrity MAPK Pathway by Zymolyase Depends on Protease and Glucanase Activities and Requires the Mucin-like Protein Hkr1 but Not Msb2

Yeast adaptation to conditions in which cell wall integrity is compromised mainly relies on the cell wall integrity (CWI) mitogen-activated protein kinase (MAPK) pathway. Zymolyase, a mixture of cell wall-digesting enzymes, triggers a peculiar signaling mechanism in which activation of the CWI pathway is dependent on the high-osmolarity glycerol MAPK pathway. We have identified inhibitors of the principal enzyme activities present in zymolyase and tested their effect on the activation of the MAPK of the CWI pathway, Slt2/Mpk1. Eventually, only β-1,3-glucanase and protease activities were essential to elicit Slt2 activation and confer lytic power to zymolyase. Moreover, we show that the osmosensor Hkr1 is required for signaling, being the most upstream element identified to date.