The membrane skeleton of Pseudomicrothorax: I. isolation, structure and composition

The epiplasm membrane skeleton of the ciliated protozoan Pseudomicrothorax dubius has been isolated and its three-dimensional structure and constituent proteins have been examined. The epiplasm functions as a cytoskeleton to define cell shape and the position of some cortical organelles. Scanning electron microscopy of the isolated epiplasm reveals a rococo cytoarchitecture in which basal bodies and trichocyst attachment sites are arranged in precise geometric arrays. SDS-PAGE reveals 40 bands, one of which is quantitatively the major band of the epiplasm and is composed of at least 3 different proteins and numerous isoelectric variants, as revealed by two-dimensional electrophoresis and peptide mapping. Polyclonal antisera were produced against native (antiserum 15) and SDS-denatured (antiserum 18) epiplasm. On immunoblots, antiserum 15 labels the hydrophilic proteins that are extracted from the epiplasm by treatment with dilute acid solution and that are predominantly glycoproteins, four of which are labeled with Concanavalin A on Western blots. On Lowicryl thin sections, antiserum 15 labels the epiplasm uniformly, except for the terminal plates, indicating that the glycoproteins are integral components of the epiplasm and are not membrane contaminants in the epiplasm fraction. Concanavalin A labeling of Lowicryl sections supports the latter result. On immunoblots, antiserum 18 labels the acid-insoluble epiplasm bands, the major structural elements of the epiplasm. One of the epiplasm bands at 52x1O3Mr is labeled by an anti-β tubulin monoclonal antibody. Evidence is presented that this β tubulin is not due to microtubule contamination of the epiplasm fraction.