Identification and Properties of Plateins,Major Proteins in the Cortical Alveolar Plates of Euplotes1

Morphogenesis of the ciliate cortex has been viewed as an attractive model system for studying the mechanisms behind the ordered assembly of subcellular structure. Based on the assumption that identifying protein components of the cortex would facilitate the study of cortical assembly, I have produced a number of monoclonal antibodies directed against components of the cortex of Euplotes aediculatus. Several of these antibodies react with the proteins comprising the alveolar plates. These thin polygonal scales, each enclosed within a flattened membranous sac (alveolus) just beneath the cell membrane, tightly abut in a confluent monolayer that appears to lend form and rigidity to the Euplotes cell cortex. Reactivity and specificity of these monoclonal antibodies for the alveolar plates was shown by immunofluorescence staining of whole-cell preparations and of cryosections, and by immuno-gold staining of thin sections by electron microscopy. On immunoblots of SDS-PAGE separated whole-cell extracts, the plate proteins are revealed as two to three closely spaced bands centered at an M_r of 97 kDa, and a larger relative at 125 kDa. Comparative peptide mapping reveals that the members of the 97-kDa protein cluster are closely related. However, the 125-kDa polypeptide varies significantly from the 97-kDa members, and hence is not likely a synthetic precursor. Because bands of these M_r values are prominent in Coomassie blue-stained gels of whole-cell extracts, and are greatly enriched in purified cortical preparations, they likely represent the major proteins comprising the alveolar plates of E. aediculatus. I have proposed the name platein for this family of proteins.