A model of intracellular electrical sorting of enzymes and organelles in the cytosol, based on isoelectric focusing, is proposed. The focusing is suggested to take place over a centrally symmetric pH gradient which in the cytosol of the yeast Saccharomyces cerevisiae is known to be 7.2-6.4. From published data on the energetic capacity and from the computed electric resistance of the S. cerevisiae cell, the maximum value of the electric field that can be maintained in the cytosol was estimated. The results showed that the strength of a centrally symmetric intracytosolic electric field could be as high as 90 mV/cm, which is sufficient to account for sorting of cytosolic proteins according to their isoelectric points. Although direct experimental evidence for intracellular isoelectric focusing is still missing, several phenomena of physiological importance can be understood on the assumption of its real existence.

Institute of Molecular Genetics Czechoslovak Academy of Sciences Prague

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A possible role of intracellular isoelectric focusing in the evolution of eukaryotic cells and multicellular organisms