ION SELECTIVITY IN THE CHLAMYDOMONAS-REINHARDTII FLAGELLAR REGENERATION SYSTEM

Extracellular Ca2+ mediates the cellular and molecular responses to ce ll stimulation of Chlamydomonas reinhardtii. Extracellular Ca2+ concen trations ([Ca2+](e)) must exceed certain threshold values to support f lagellar excision by acid shock and to stimulate flagellar outgrowth f ollowing mechanical shear of the flagella. Also, the magnitude and dur ation of flagellar RNA accumulations following acid shock or mechanica l shear increase with increasing [Ca2+](e). To better understand the r ole of Ca2+ in flagellar excision, RNA induction, and outgrowth, we ha ve performed a survey of the ion selectivity of each of these response s to acid shock. We found that flagellar excision in vivo following ac id shock was supported by Sr2+ and Ca2+, but no other ion tested. LaCl 3 and neomycin prevented flagellar excision following acid shock of ce lls in Ca2+- or Sr2+-containing buffer. Sr2+ addition to detergent-per meabilized cell models, however, failed to elicit flagellar excision i n vitro. Cells failed to regrow flagella following flagellar excision in Sr2+-containing buffer unless exogenous Ca2+ was added. Flagellar R NA accumulations of lower magnitude and shorter duration were measured in cells acid-shocked in Sr2+- containing buffer than in Ca2+-contain ing buffer. These results demonstrate that a Sr2+ influx can evoke fla gellar excision following acid shock, but cannot directly activate the machinery for flagellar excision, suggesting that a Sr2+ influx induc es excision by stimulating an intracellular Ca2+ release. Furthermore, they suggest that flagellar outgrowth and normal flagellar RNA induct ion have a strict requirement for Ca2+, which is not satisfied by the proposed intracellular Ca2+ release. (C) 1997 Academic Press