Phosphorylation in isolated XXXX axonemes: a phosphoprotein may mediate the Ca2+-dependent photophobic response.

@entity460 @entity2060
An in vitro system was devised for studying phosphorylation of @entity2060 axonemal proteins. Many of the polypeptides phosphorylated in this system could be identified as previously described axonemal components that are phosphorylated in vivo. The in vitro system apparently preserved the activities of diverse axonemal kinases without greatly altering the substrate specificity of the enzymes. The in vitro system was used to study the effect of @entity460 concentration on axonemal protein phosphorylation. @entity460 has previously been demonstrated to initiate the axonemal reversal reaction of the photophobic response; the in vitro system made it possible to investigate the possibility that this @entity460 effect is mediated by protein phosphorylation. @entity460 specifically altered the phosphorylation of only two axonemal proteins; the phosphorylation of an otherwise unidentified 85,000 Mr protein was repressed by @entity460 concentrations greater than or equal to 10(-6) M, while the phosphorylation of the previously identified 95,000 Mr protein b4 was stimulated by @entity460 at concentrations greater than 10(-6) M. Protein b4 is one of six polypeptides that are deficient in the mbo mutants, strains that do not exhibit a photophobic reversal reaction. Therefore, this @entity460 -stimulated phosphorylation may be involved in initiating the photophobic response. Neither calmodulin nor the C-kinase could be implicated in b4 phosphorylation. The @entity460 -dependent activation of the b4 kinase was not affected by several drugs that bind to and inhibit calmodulin, or by the addition of exogenous calmodulin. Activators and inhibitors of the @entity460 -phospholipid-dependent C kinase also had no effect on b4 phosphorylation.
