The cytoskeleton of certain protists comprises an extensive membrane skeleton, the epiplasm, which contributes to the cell shape and patterning of the species-specific cortical architecture. The isolated epiplasm of the ciliated protist Pseudomicrothorax dubius consists of two major groups of proteins with molecular masses of 78-80 kD and 11-13 kD, respectively. To characterize the structure of these proteins, peptide sequences of two major polypeptides (78-80 kD) as well as a cDNA representing the entire coding sequence of a minor and hitherto unidentified component (60 kD; p60) of the epiplasm have been determined. All three polypeptides share sequence similarities. They contain repeated valine- and proline-rich motifs of 12 residues with the consensus VPVP--V-V-V-. In p60 the central core domain consists of 24 tandemly repeated VPV motifs. Within the repeat motifs positively and negatively charged residues, when present, show an alternating pattern in register with the V and P positions. Recombinant p60 was purified in 8 M urea and dialyzed against buffer. Infrared spectroscopic measurements indicate 30% beta-sheet. Electron microscopy reveals short filamentous polymers with a rather homogenous diameter (approximately 15-20 nm), but variable lengths. The small polymers form thicker filaments, ribbons, and larger sheets or tubes. A core domain similar to that of P. dubius p60 is also found in the recently described epiplasmic proteins of the flagellate Euglena, the so-called articulins. Our results show that the members of this protein family are not restricted to flagellates, but are also present in the distantly related ciliates where they are major constituents of the epiplasm. Comparison of flagellate and ciliate articulins highlights common features of this novel family of cytoskeletal proteins.

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