Ca2+ influx activated by low pH in Chlamydomonas

Abstract

Cystolic acidification stimulates an influx of Ca2+ which results in shedding of the two flagella of Chlamydomonas. Ca2+ influxes are also involved in the photoresponses of this alga, but it is not understood how the acidification-activated Ca2+ influx is distinguished from the Ca2+ influxes which mediate phototaxis and the photophobic response. The present study focuses on the deflagellation-inducing Ca2+ influx pathway. Influx occurs through an ion channel or transporter with low abundance or low permeability to Ca2+ (~500 fmol/s/106 cells in 50 μM Ca2+). Ca2+ influx was potentially blocked by Gd3+ (EC50 ~5 μM), but was insensitive to Cd2+ (Quarmby, L.M. and H.C. Hartzell, 1994, J. Cell Biol. 124;807) and organic blockers of Ca2+ channels including SKF-96365 (up to 100 μM) and flufenamic acid (up to 1 mM). Experiments with a flagella-less mutant (bald-2), isolated flagella, and a blocker of flagellar assembly (colchicine) indicated that the acidification-stimulated Ca2+ influx pathway is not localized to the flagellar membrane. The acid-stimulated influx pathways was transiently inactivated after cells shed their flagella. Inactivation did not occur in the deflagellation mutant, fa-1, although acidification-stimulated Ca2+ influx was normal. This suggests that inactivation of this pathway in wild-type cells is probably not a direct consequence of acidification nor of Ca2 influx, but may be related to deflagellation. Recovery of deflagellation-inducing Ca2+ influx occurred within 30 min after a 30 s exposure to acid and did not require flagellar assembly. The regulation, drug sensitivity, and subcellular localization identify acidification-stimulated Ca2+ influx as a specific Ca2+ entry pathway distinct from established Ca2+ channels.