Characterization of a protein kinase (FsPK4) with an acidic domain, regulated by abscisic acid and specifically located in Fagus sylvatica L. seeds.

An abscisic acid (ABA)-induced cDNA fragment encoding a putative serine/threonine protein kinase (PK) was obtained by means of differential reverse transcriptase-polymerase chain reaction (RT-PCR). The full-length clone (FsPK4) was isolated from a cDNA library constructed using mRNA from ABA-treated Fagus sylvatica L. seeds. This clone contained the 11 catalytic domains present in all PKs and a highly acidic domain in the C-terminus. By expressing FsPK4 in Escherichia coli as a His tag fusion protein, we obtained direct biochemical evidence supporting Ca2+-dependent kinase activity of this protein. The expression of FsPK4 increased after ABA treatment or warm pre-treatment, when seeds are maintained dormant, but decreased and tended to disappear when dormancy was released by stratification or under gibberellic acid (GA3) treatment, and when seeds were artificially dried. Further, FsPK4 transcript expression is tissue specific, and was found to accumulate in ABA-treated seeds rather than in other ABA-treated vegetative tissues examined. These results suggest that the expression of the corresponding protein could be more closely related with the maintenance of seed dormancy than with responses to drought stress mediated by ABA.