COVISUALIZATION IN LIVING ONION CELLS OF PUTATIVE INTEGRIN, PUTATIVE SPECTRIN, ACTIN, PUTATIVE INTERMEDIATE FILAMENTS, AND OTHER PROTEINS AT THE CELL-MEMBRANE AND IN AN ENDOMEMBRANE SHEATH

Covisualizations with wide-field computational optical-sectioning micr oscopy of living epidermal cells of the onion bulb scale have evidence d two major new cellular features. First, a sheath of cytoskeletal ele ments clads the endomembrane system. Similar elements clad the inner f aces of punctate plasmalemmal sites inter interpreted as plasmalemmal control centers. One component of the endomembrane sheath and plasmale mmal control center cladding is antigenicity-recognized by two injecte d antibodies against animal spectrin. Immunoblots of separated epiderm al protein also showed bands recognized by these antibodies. Injected phalloidin identified F-actin with the same cellular distribution patt ern, as did antibodies against intermediate-filament protein and other cytoskeletal elements known from animal cells. Injection of general p rotein stains demonstrated the abundance of endomembrane sheath protei n. Second, the endomembrane system, like the plasmalemmal puncta, cont ains antigen recognized by an anti-beta(1) integrin injected into the cytoplasm. Previously, immunoblots of separated epidermal protein were shown to have a major band recognized both by this antibody prepared against a peptide representing the cytosolic region of beta(1) integri n and an antibody against the matrix region of beta(1) integrin. The l atter antibody also identified puncta at the external face of protopla sts. It is proposed that integrin and associated transmembrane protein s secure the endomembrane sheath and transmit signals between it and t he lumen or matrix of the endoplasmic reticulum and organellar matrice s. This function is comparable to that proposed for such transmembrane linkers in the plasmalemmal control centers, which also appear to bin d cytoskeleton and a host of related molecules and transmit sig nals b etween them and the wall matrix. It is at the plasmalemmal control cen ters that the endoplasmic reticulum, a major component of the endomemb rane system, attaches to the plasma membrane.