Demonstration of a 58kd protein as a major component of the plasma membrane skeleton in the ciliateEntodinium bursa

Abstract

InEntodinium bursa, proteins of isolated cortices have been analysed by gel electrophoresis. Besides tubulins, several major protein components with M.W. comprised between 21,000 and 195,000 were resolved. Treatment of cortices under nitrogen pressure facilitated the purification of the membrane skeleton also termed the epiplasm to which large fragments of plasma membrane remain associated. The major constituent of such fractions was a single molecular species resembling tubulins in its mobility on one and twodimensional gels. The relationship of this polypeptide to tubulin subunits was examined and differences could be established by distinct peptide maps and antigenic properties. Immunoelectron microscopy was used to show that this polypeptide was closely associated with the entire epiplasmic layer. This technique also revealed its apparent exclusion from other cortical structures such as the plasma membrane or sub-epiplasmic microtubules. Its insolubility and relative abundance within epiplasm-enriched fractions render this protein a likely candidate for the class of 3-nm filament proteins which may constitute the bulk of the plasma membrane skeleton in a variety of protists.