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The physico-chemical properties of lipid components isolated from zoospores of the aquatic phycomycete, Blastocladiella emersonii, were investigated with electron spin resonance (ESR) spectroscopy using the spin label, 5-nitroxystearate. Lipid dispersions were made from zoospore phospholipids and glycolipids, both singly and in combination with each other and with isolated neutral lipid components. Plots of the hyperfine splitting parameter (2T parallel) vs. temperature indicate that it is the zoospore glycolipids rather than the phospholipids which are responsible for the phase transformations previously observed in aqueous dispersions of the total lipids extracted from zoospores and in zoospores in vivo. The discontinuities observed in the glycolipid dispersions seem to represent the onset and completion of a gel-to-liquid-crystalline phase transition. Over the temperature range tested, Ca2+ increased the rigidity of the glycolipid dispersions, the major component of which is probably a diglucosyldiglyceride, but had no effect on the phospholipid dispersions. The increase in 2T parallel was not affected by inclusion of neutral lipids into the glycolipid dispersion but was eliminated at high (5 : 1, w/w) phospholipid-to-glycolipid ratios. The Ca2+ effect was relatively independent of both the absolute rigidity of the dispersion and its phase (gel or liquid-crystalline), suggesting an interaction with the glycolipid head group rather than the hydrocarbon core. The Ca2+-induced increase in 2T was neither prevented nor reversed by the presence of K+. The presence of two spin label populations co-existing in a dynamic equilibrium was found in glycolipid/neutral lipid dispersions. Plots of the percentage ([HA/(HA + HB)] X 100 of the spin label population, as measured by the peak height of the low-field peaks, corresponding to the more immobilized component (HA) vs. temperature indicated two break points. The temperatures at which these break points occurred are similar to those obtained for the glycolipid dispersions, and match the break points (TL and TH) found in ESR experiments using zoospores in vivo. The importance of the glycolipids in the development of this organism is discussed.

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.

The physical-chemical properties of the isolated plasma membranes from zoospores of the chytridiomycete Blastocladiella emersonii were investigated, with electron spin resonance (ESR) spectroscopy, using the spin-label 5-nitroxystearate (5-NS). Both isolated plasma membranes and aqueous dispersion of the lipids extracted from the plasma membranes were spin-labeled and analyzed. Plots of the hyperfine splitting parameter (2T) vs. temperature indicated that the middle break point, TM, initially observed in experiments with spin-labeled zoospores in vivo [Leonards, K. S., & Haug, A. (1980) Biochim. Biophys. Acta 600, 805-816], was the result of a lipid-lipid interaction (glycolipid-glycolipid or glycolipid-neutral lipid) rather than a lipid-protein interaction. This interaction was markedly affected by Ca2+ ions, which interacted directly with the lipid components, increasing TM from 11 +/- 1 (Ca2+ removed by EDTA) to 21 +/- 1 degree C (10 mM Ca2+) in the lipid dispersions and from 12 +/- 1 to 23 +/- 1 degree C in the plasma membrane preparations. The initial ESR studies on spin-labeled zoospores in vivo had also demonstrated that the addition of K+ ions could reverse the Ca2+ ion effect, downshifting TM from 22 +/- 1 to 10 +/- 1 degree C. The addition of of K+ ions to the isolated plasma membrane had no affect on TM, indicating that K+ ions do not simply replace Ca2+ ions but exert their effect indirectly on the membrane. However, after the inclusion of ATP, K+ ions could reverse the Ca2+ ion effect. it was determined that the ATp generated an "energized membrane" state which permitted the K+ ions to reverse the Ca2+ effect. Since K+ ions have been shown to depolarize the membrane potential in both zoospores and isolated zoospore plasma membrane preparations (generated by ATP), were suggest that the K+ ion induced reversal of the Ca2+ ion effect, and therefore the change in the lipid-lipid interactions responsible for TM, is a consequence of the K+ ion induced depolarization of the membrane potential.