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Eukaryotic Cell, November 2005, p. 1794-1800, Vol. 4, No. 11
1535-9778/05/$08.00+0     doi:10.1128/EC.4.11.1794-1800.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Cyclic AMP-Independent Regulation of Protein Kinase A Substrate Phosphorylation by Kelch Repeat Proteins

Ailan Lu and Jeanne P. Hirsch*

Department of Pharmacology and Biological Chemistry, Mount Sinai School of Medicine, New York, New York 10029

Received 27 July 2005/ Accepted 27 August 2005

Pseudohyphal and invasive growth in the yeast Saccharomyces cerevisiae is regulated by the kelch repeat-containing proteins Gpb1p and Gpb2p, which act downstream of the G protein {alpha}-subunit Gpa2p. Here we show that deletion of GPB1 and GPB2 causes increased haploid invasive growth in cells containing any one of the three protein kinase A (PKA) catalytic subunits, suggesting that Gpb1p and Gpb2p are able to inhibit each of these kinases. Cells containing gpb1{Delta} gpb2{Delta} mutations also display increased phosphorylation of the PKA substrates Sfl1p and Msn2p, indicating that Gpb1p and Gpb2p are negative regulators of PKA substrate phosphorylation. Stimulation of PKA-dependent signaling by gpb1{Delta} gpb2{Delta} mutations occurs in cells that lack both adenylyl cyclase and the high-affinity cyclic AMP (cAMP) phosphodiesterase. This effect is also seen in cells that lack the low-affinity cAMP phosphodiesterase. Given that these three enzymes control the synthesis and degradation of cAMP, these results indicate that the effect of Gpb1p and Gpb2p on PKA substrate phosphorylation does not occur by regulating the intracellular cAMP concentration. These findings suggest that Gpb1p and Gpb2p mediate their effects on the cAMP/PKA signaling pathway either by inhibiting the activity of PKA in a cAMP-independent manner or by activating phosphatases that act on PKA substrates.

* Corresponding author. Mailing address: Department of Pharmacology and Biological Chemistry, Mount Sinai School of Medicine, Box 1603, 1 Gustave L. Levy Place, New York, NY 10029. Phone: (212) 241-0224. Fax: (212) 996-7214. E-mail: jeanne.hirsch{at}mssm.edu.

Eukaryotic Cell, November 2005, p. 1794-1800, Vol. 4, No. 11
1535-9778/05/$08.00+0     doi:10.1128/EC.4.11.1794-1800.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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