Phosphorylation of algal centrin is rapidly responsive to changes in the external milieu.

Centrin, a calcium-sensitive contractile phosphoprotein, is the major component of the striated flagellar roots of the flagellate green alga, Tetraselmis striata. Flagellar roots contract in response to elevated calcium levels. Data presented here indicate that the level of centrin phosphorylation is rapidly responsive to changes in the cell's external environment. Centrin is dephosphorylated in response to elevated calcium or to heat shock. An increase in centrin phosphorylation accompanies pH shock or ethanol treatment. These changes are compared with flagellar excision, flagellar root contraction, and protein synthesis under the same treatments. We conclude that under certain conditions phosphorylation/dephosphorylation of centrin and extension/contraction of the flagellar root are uncoupled. Possible role(s) of centrin phosphorylation independent of force generation in the flagellar root are discussed.