Signal transduction pathways regulating cortical microtubule functions

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Many extragenic suppressors were obtained after EMS-mutagenesis of the semi-dominant phs1-1 allele. A group of suppressors did not follow a Mendelian segregation and could not be mapped in this study. Other suppressors were shown to have mis-sense mutations in Arabidopsis tubulin genes.Full length PHS1 cDNAs of wild type and the phs1-1-type mutantation were used to screen Arabidopsis cDNA libraries by a yeast two-hybrid method, bud did not result in positive clones.When a phosphatase-dead mutant of PHS1 was expressed in plant cells, it strongly induced depolymerization of cortical microtubules. This dominant-negative effect was specific to PHS1 since similar phosphatase-dead mutants of other four Arabidopsis MAP kinase phosphatases did not affect cortical microtubule organization. PHS1 is positively retained in the cytoplasm. A C-terminal region excluding the phosphatase catalytic domain was necessary and sufficient for the cytoplasmic retention. Possibly, PHS1 may dephosporylate microtubule-related factors in the cytoplasm.While screening downstream targets of PHS1, we found that Ser-172 of β-tubulin is phosphorylated, cyclin-dependent kinases catalyze this phosphorylation, and the phosphorylated b-tubulin significantly accumulates in an Arabidopsis ton2 mutant of a regulatory subunit in the PP2A phosphatase.