CAEGTA UNCOMPETITIVELY INHIBITS CALCIUM ACTIVATION OF WHORL MORPHOGENESIS IN ACETABULARIA

The spacing between adjacent hairs in vegetative whorls of Acetabulari a acetabulum (formerly A. mediterranea) was earlier reported as being quantitatively responsive to calcium ion concentration in the culture medium. We here report a quantitative response to the concentration of the calcium-chelator EGTA, in the opposite sense to the effect of cal cium. (Increasing [Ca2+] diminishes the spacing, increasing [EGTA] inc reases it.) The earlier work was interpreted in terms of control of th e spacing by a putative reaction-diffusion mechanism in the cell membr ane, in which a receptor R was activated by calcium-binding to initiat e the process. We extend this interpretation by treating CaEGTA as an uncompetitive inhibitor of the effect of calcium on R. This leads to t hermodynamic constants for CaEGTA binding to the CaR complex: Delta H- 298(0) = -250 +/- 60 kJ/mol; Delta S-298(0) = -820 +/- 200 J/mol . K. Consistency of the concentration and. temperature dependences reported here with the postulated dynamic mechanism increases the probability that this mechanism is correct.