Phosphorylation of endogenous proteins of cilia from Paramecium tetraurelia in vitro

Several endogenous substrate proteins of cilia from axenically grown Paramecium tetraurelia were phosphorylated in vitro by inherent protein kinases (PKs). Labeling was stimulated by cAMP and to a lesser extent by cGMP. ATP breakdown was most rapid in cilia and subciliary fractions. Using multiple substrate additions during incubations it was shown that phosphorylation was almost completed within 30 s. Very little dephosphorylation by phosphoprotein phosphatases occurred during 5 min of incubation. Proteins of molecular weight of 103 000 and 46 000 were shown to be particularly associated with axonemal structures of the cilia. No distinct differences in phosphorylation patterns were apparent in ciliary membrane vesicles of low and high buoyant density, which exhibit differential enzyme patterns. cAMP receptor proteins were identified by use of the photoaffinity label 8-azido-[32P]cAMP. Receptor proteins with apparent molecular weights of 43 000, 39 000, 37 000, 31 000 and 30 000 were probably related to the regulatory subunits of cAMP-dependent protein kinases as evidenced by inhibition of incorporation of the photoaffinity label by low concentrations of cAMP. Tagging of a protein of 85 000 molecular weight was specifically inhibited by cGMP, thus in all likelihood it corresponded to a cGMP-dependent protein kinase. Corresponding autophosphorylated protein bands were observed with gamma-[32P]ATP. A functional role for protein phosphorylation in cilia of Paramecium remains to be established.