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A more recent version of this article appeared on September 1, 2002
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Submitted on April 12, 2002
Revised on May 28, 2002
Accepted on June 5, 2002
1 Department of Molecular and Cell Biology, Division of Biochemistry and Molecular Biology, University of California, Berkeley, CA 94720-3202
2 Department of Molecular and Cell Biology, Division of Biochemistry and Molecular Biology, University of California, Berkeley, CA 94720-3202 (present address: GeneLabs Technologies, Inc., Redwood City, CA 94603)
3 Department of Molecular and Cell Biology, Division of Biochemistry and Molecular Biology, University of California, Berkeley, CA 94720-3202 (present address: Sangamo Biosciences, Inc., Richmond, CA 94804)
* Corresponding author. E-mail address: jeremy{at}socrates.berkeley.edu.
S. cerevisiae Pkh1 and Pkh2 are functionally redundant homologs of mammalian protein kinase, PDK1. They activate two closely-related, functionally redundant enzymes, Ypk1 and Ykr2 (homologs of mammalian protein kinase, SGK). We found that Ypk1 has a more prominent role than Ykr2 in mediating their shared essential function. Considerable evidence demonstrated that Pkh1 preferentially activates Ypk1, whereas Pkh2 preferentially activates Ykr2. Loss of Pkh1 (but not Pkh2) reduced Ypk1 activity; conversely, Pkh1 overexpression increased Ypk1 activity more than Pkh2 overexpression. Loss of Pkh2 reduced Ykr2 activity; correspondingly, Pkh2 overexpression increased Ykr2 activity more than Pkh1 overexpression. When overexpressed, a catalytically-active C-terminal fragment (kinase domain) of Ypk1 was growth-inhibitory; loss of Pkh1 (but not Pkh2) alleviated toxicity. Loss of Pkh2 (but not Pkh1) exacerbated the slow growth phenotype of a ypk1
strain. This Pkh1-Ypk1 and Pkh2-Ykr2 dichotomy is not absolute because all double mutants (pkh1 ypk1, pkh2 ypk1, pkh1 ykr2, and pkh2 ykr2) were viable. Compartmentation contributes to selectivity because Pkh1 and Ypk1 were located exclusively in the cytosol, whereas Pkh2 and Ykr2 entered the nucleus. At restrictive temperature, ypk1-1ts ykr2 cells lysed rapidly, but not in medium containing osmotic support. Dosage and extragenic suppressors were selected. Overexpression of Exg1 (major exoglucanase), or loss of Kex2 (endoprotease involved in Exg1 processing), rescued growth at high temperature. Viability was also maintained by PKC1 overexpression or an activated allele of the downstream protein kinase (BCK1-20). Conversely, absence of Mpk1 (distal MAP kinase of the PKC1 pathway) was lethal in ypk1-1ts ykr2 cells. Thus, Pkh1-Ypk1 and Pkh2-Ykr2 function in a novel pathway for cell wall integrity that acts in parallel with the Pkc1-dependent pathway.
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