Effects of cations of the plasma membrane of Blastocladiella emersonii zoospores An ESR study.

The physical properties of the plasma membrane of the aquatic phycomycete Blastocladiella emersonii were investigated, in particular the effects of cations on membrane structure. Intact zoospores and lipid extracts were labelled with the spin-labels 5-nitroxystearate (5-NS), 12-nitroxystearate (12-NS), and 2,2,6,6-tetramethylpiperidine-1-oxyl (Tempo). Electron spin resonance spectroscopy indicated a total of three breaks in plots of the hyperfine splitting parameter, 2T parallel, order parameter, S, and the partition coefficient, f, vs. temperature. The first and third break points (TL and TH) were found to be independent of the external K+, Ca2+, or Mg2+ concentrations. They were similar to the break points found in aqueous dispersions of lipid extracts and correlate well with the temperature limits for zoospore liability. In contrast, the middle break point (TM) was markedly influenced by the external Ca2+ concentration. Ca2+ increased TM from 12 degrees C (no Ca2+ added) to 22 degrees C (10 mM Ca2+), i.e., growth temperature. K+ reversed this Ca2+ effect, downshifting TM from 22 degrees C to 10 degrees C. A comparison of the physico-chemical effects of these ions on the membrane, as revealed by the cation-induced shift in TM, is closely correlated with the temperature dependence and physiological effects of cations on zoospore differentiation. This suggest that cations may modify the physical state of the plasma membrane and be involved in regulating the initial changes during zoospore encystment.