Chlamydomonas reinhardtii cells shed their flagella in response to environmental stress. Under favorable conditions, flagella are quickly regrown. To learn more about the signals that trigger flagellar excision and regrowth we have investigated inositol phospholipid metabolites, molecules implicated in signal transduction in several other systems. After deflagellation by low pH or mastoparan, a potent activator of G proteins, there was a rapid increase in levels of inositol 1,4,5-trisphosphate measured by use of receptor-binding assays and HPLC. This increase was concomitant with a decrease in levels of phosphatidylinositol 4,5-bisphosphate and was followed by an increase in phosphatidic acid, results consistent with activation of phospholipase C and diacylglycerol kinase. Additional experiments suggest that this activated phospholipase C is not important for flagellar regrowth but plays a role in informing the excision apparatus of the environmental stress. Addition of neomycin (an inhibitor of phospholipase C) before exposure of cells to low pH or mastoparan prevented the increase in inositol 1,4,5-trisphosphate and also prevented deflagellation. Addition of neomycin after deflagellation blocked increases in inositol 1,4,5-trisphosphate that normally followed deflagellation, but did not block flagellar assembly. Furthermore, a flagellar excision-defective mutant, fa-1, did not shed its flagella in response to low pH or mastoparan, yet both of these agents activated phospholipase C in these cells. The results suggest that activation of phospholipase C, possibly via a G protein, is a proximal step in the signal transduction pathway inducing deflagellation in Chlamydomonas.

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