Enzymatic form and cytoskeletal form of bifunctional Tetrahymena 49kDa protein is regulated by phosphorylation.

Tetrahymena 49kDa protein functions as a citrate synthase (CS) and also assembles to 14-nm filament during cell mating. Bifunctional property of 49kDa protein is suggested to be maintained by the difference of post-translational modification(s). We have found that phosphorylation is present on all three isoforms of 49kDa protein. Dephosphorylation of citrate synthase type isoforms of 49kDa protein, composing pl 7.7 and 8.0 isoforms, reduced its enzymatic activity, shifting these isoforms to basic side. In a course of dephosphorylation, isoform of pl 8.4 appeared with pl 7.7 and 8.0 isoforms, which correspond to the isoforms of 14-nm filament assembling type. With this dephosphorylation, the citrate synthase type isoforms obtained the ability to assemble 14-nm filaments. We propose that enzyme form and cytoskeletal form of 49kDa protein were maintained simply by phosphorylation.