Ca2+ influx activated by low pH in Chlamydomonas.

Cytosolic acidification stimulates an influx of Ca 2+ which results in shedding of the two flagella of Chlamydomonas. Ca `)+ influxes are also involved in the photoresponses of this alga, but it is not understood how the acidification-activated Ca 2§ influx is distinguished from the Ca 2+ influxes which mediate phototaxis and the pho- tophobic response. The present study focuses on the deflagellation-inducing Ca `)+ influx pathway. Influx occurs through an ion channel or transporter with low abundance or low permeability to Ca `)+ (,'~500 fmol/s/106 cells in 50 txM Ca`)+). Ca `)+ influx was potently blocked by Gd 3+ (ECs0 ~5 IxM), but was insensitive to Cd 2+ (Quarmby, L.M., and H.C. Hartzetl. 1994. J. Cell Biol. 124:807) and organic blockers of Ca `)+ channels including SKF-96365 (up to 100 p~M) and flufenamic acid (up to 1 mM). Experiments with a flagella-less mutant (bald-2), isolated fla- gella, and a blocker of flagellar assembly (colchicine) indicated that the acidification-stimulated Ca 2+ influx path- way is not localized to the flagellar membrane. The acid-stimulated influx pathway was transiently inactivated after cells shed their flagella. Inactivation did not occur in the deflagellation mutant, fa-1, although acidification-stimu- lated Ca 2+ influx was normal. This suggests that inactivation of this pathway in wild-type cells is probably not a di- rect consequence of acidification nor of Ca 2+ influx, but may be related to deflagellation. Recovery of deflagella- tion-inducing Ca 2+ influx occurred within 30 rain after a 30 s exposure to acid and did not require flagellar assem- bly. The regulation, drug sensitivity, and subcellular localization identify acidification-stimulated Ca `)+ influx as a specific Ca `)+ entry pathway distinct from established Ca z+ channels.