Abstract: The influence of cAMP-dependent protein kinase (PKA) on protein expression during exponential growth under osmotic stress was studied by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). The responses of isogenic strains (tpk2Deltatpk3Delta) with either constitutively low (tpk1(w1)), regulated (TPK1) or constitutively high (TPK1bcy1Delta) PKA activity were compared. The activity of cAMP-dependent protein kinase (PKA) was shown to be a major determinant of osmotic shock tolerance. Proteins with increased expression during growth under sodium chloride stress could be grouped into three classes with respect to PKA activity, with the glycerol metabolic proteins GPD1, GPP2 and DAK1 standing out as independent of PKA. The other osmotically induced proteins displayed a variable dependence on PKA activity; fully PKA-dependent genes were TPS1 and GCY1, partly PKA-dependent genes were ENO1, TDH1, ALD3 and CTT1. The proteins repressed by osmotic stress also fell into distinct classes of PKA-dependency. Ymr116c was PKA-independent, while Pgi1p, Sam1p, Gdh1p and Vma1p were fully PKA-dependent. Hxk2p, Pdc1p, Ssb1p, Met6p, Atp2p and Hsp60p displayed a partially PKA-dependent repression. The promotors of all induced PKA-dependent genes have STRE sites in their promotors suggestive of a mechanism acting via Msn2/4p. The mechanisms governing the expression of the other classes are unknown. From the protein expression data we conclude that a low PKA activity causes a protein expression resembling that of osmotically stressed cells, and furthermore makes cells tolerant to this type of stress. Copyright 2000 John Wiley & Sons, Ltd.