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T-Loop Phosphorylation of Arabidopsis CDKA;1 Is Required for Its Function and Can Be Partially Substituted by an Aspartate Residue ^{[W] ,[OA]}

^a University of Cologne, University Group at the Max Planck Institute for Plant Breeding Research, Max Delbrück Laboratory, Department of Botany III, 50829 Cologne, Germany
^b Department of Plant Systems Biology, Flanders Interuniversity Institute for Biotechnology, Ghent University, B-9052 Gent, Belgium
^c University of Oslo, Department of Molecular Biosciences, N-0316 Oslo, Norway

² To whom correspondence should be addressed. E-mail schnitt{at}mpiz-koeln.mpg.de; fax 49-221-5062-113.

As in other eukaryotes, progression through the cell cycle in plants is governed by cyclin-dependent kinases. Phosphorylation of a canonical Thr residue in the T-loop of the kinases is required for high enzyme activity in animals and yeast. We show that the Arabidopsis thaliana Cdc2⁺/Cdc28 homolog CDKA;1 is also phosphorylated in the T-loop and that phosphorylation at the conserved Thr-161 residue is essential for its function. A phospho-mimicry T161D substitution restored the primary defect of cdka;1 mutants, and although the T161D substitution displayed a dramatically reduced kinase activity with a compromised ability to bind substrates, homozygous mutant plants were recovered. The rescue by the T161D substitution, however, was not complete, and the resulting plants displayed various developmental abnormalities. For instance, even though flowers were formed, these plants were completely sterile as a result of a failure of the meiotic program, indicating that different requirements for CDKA;1 function are needed during plant development.

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