Protein phosphatases PP1 and PP2A are located in distinct positions in theChlamydomonas flagellar axoneme

We postulated that microcystin-sensitive protein phosphatases are integral components of the Chlamydomonas flagellar axoneme, positioned to regulate i nner arm dynein activity. To test this, we took a direct biochemical approa ch. Microcystin-Sepharose affinity purification revealed a prominent 35-kDa axonemal protein, predicted to be the catalytic subunit of type-1 protein phosphatase (PP1c), We cloned the Chlamydomonas PP1c and produced specific polyclonal peptide antibodies. Based on western blot analysis, the 35-kDa P P1c is anchored in the axoneme, Moreover, analysis of flagella and axonemes from mutant strains revealed that PP1c is primarily, but not exclusively, anchored in the central pair apparatus, associated with the C1 microtubule, Thus, PP1 is part of the central pair mechanism that controls flagellar mo tility, Two additional axonemal proteins of 62 and 37 kDa were also isolate d using microcystin-Sepharose affinity. Based on direct peptide sequence an d western blots, these proteins are the A- and C-subunits of type 2A protei n phosphatase (PP2A), The axonemal PP2A is not one of the previously identi fied components of the central pair apparatus, outer arm dynein, inner arm dynein, dynein regulatory complex or the radial spokes. We postulate PP2A i s anchored on the doublet microtubules, possibly in position to directly co ntrol inner arm dynein activity.