BIOCHEMISTRY Shortcut to PKA in Yeast

Cells usually activate the cyclic AMP (cAMP, or adenosine 3',5'-monophosphate)-dependent protein kinase through G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptors, which activate G proteins, which in turn control the activity of adenylyl cyclase, the enzyme that makes cAMP. Peeters et al., however, report that in yeast, and just maybe in mammalian cells as well, there appears to be a more direct route to activation of the cAMP-dependent protein kinase (PKA). Peeters et al. studied the roles of Krh1 (kelch-repeat homologue 1, also called Gpb2) and Krh2 (also called Gpb1). Krh1 and Krh2 associate with the yeast G protein α subunit Gpa2, which appears not to interact with canonical G protein β- subunits. Interestingly, the Krh1 and Krh2 proteins have the seven-bladed β-propeller structure characteristic of G_β proteins. Deletion of Krh1 and Krh2 resulted in a phenotype indicative of high PKA activity, but there was no associated increase in the abundance of cAMP in the cells. Furthermore deletion of Krh1 and Krh2 suppressed the poor growth of cells lacking the yeast gene encoding adenylyl cyclase, as long as a minimal amount of cAMP was present in the culture. Thus, Krh1 and Krh2 appeared to act independently of cAMP. Nevertheless, the effects of the Krh proteins depended on the yeast PKA enzymes, and cells lacking the three genes encoding PKA catalytic subunits or the BCY1 regulatory subunit gene failed to respond to loss of Krh1 and Krh2. The authors used two-hybrid methods and in vitro binding assays to show that Krh1 bound to Gpa2, but also to the PKA holoenzyme through direct interaction with the catalytic subunit. A two-hybrid reporter assay showed that interaction of the regulatory and catalytic subunits appeared to decrease in vivo when function of Krh1 and Krh2 was lost. The authors suggest that these effects may be relevant in mammals as well because interaction between mouse catalytic and regulatory subunits, substituted in this yeast two-hybrid assay, was also decreased when Krh1 and Krh2 were not present. Overall, the results suggest a mechanism in which PKA can be controlled without participation of adenylyl cyclase: A sugar-sensing receptor on the yeast cell surface activates the Gpa2 alpha subunit, which then relieves the direct inhibition of PKA by Krh1 and Krh2.

T. Peeters, W. Louwet, R. Geladé, D. Nauwelaers, J. M. Thevelein, M. Versele, Kelch-repeat proteins interacting with the G_α protein Gpa2 bypass adenylate cyclase for direct regulation of protein kinase A in yeast. Proc. Natl. Acad. Sci. U.S.A. 103, 13034-13039 (2006). [Abstract] [Full Text]