The physical state of the intracytoplasmic membrane of Rhodopseudomonas sphaeroides and its relationship to the cell division cycle.

A previous study has established that the protein to phospholipid ratio of the intracytoplasmic membranes of Rhodopseudomonas sphaeroides undergoes cyclical fluctuations during synchronous growth of this organism (Lueking, D. R., Fraley, R. T., and Kaplan, S. (1978) J. Biol. Chem. 253, 451-457). This pattern has been shown to result from large changes in the rate of phospholipid synthesis which occur during the cell cycle (Fraley, R. T., Lueking, D. R., and Kaplan, S. (1979) J. Biol. Chem. 254,1980-1986). The present study demonstrates that the observed physiological changes in membrane composition are accompanied by changes in the fluidity of the intracytoplasmic membranes, as determined by use of the fluorescent probe, cw-parinaric acid. Increasing or decreasing the relative protein content of the intracytoplasmic membrane during synchronous growth results in a corresponding change in the relative mobility of the a-parinaric acid fluorophore. The magnitude of the polarization changes (-0.025) observed during the cell cycle are equivalent to those observed in intracytoplasmic membranes upon increasing or decreasing the temperature by -20°C. It is unlikely that the observed changes in the polarization of a-parinaric acid fluorescence can be accounted for by changes in vesicle size, the cell cyclespecific interaction of the probe with unique membrane components, or variations in the fraction of the probe bound to protein during the division cycle. Instead, the changes are believed to reflect the influence of intracytoplasmic membrane proteins in modulating bulk membrane fluidity. The results indicate that the perturbing effect of protein on the mobility of membrane components should be considered as an important mechanism in the maintenance of membrane fluidity and its regulation during the cell division cycle.