A NOVEL TYPE OF ADHERING JUNCTION IN AN EPITHELIOID TUMORIGENIC RAT-CELL CULTURE LINE

Two major types of plaque-bearing adhering junctions are commonly dist inguished: the actin microfilament-anchoring adhaerens junctions (AJs) and the desmosomes anchoring intermediate-sized filaments (IFs). Both types of junction usually possess the common plaque protein, plakoglo bin, whereas the other plaque proteins and the transmembrane cadherins are mutually exclusive. For example, AJs contain E-, N-, or P-cadheri n in combination with alpha- and beta-catenin, vinculin and alpha-acti nin, whereas in desmosomes, desmogleins and desmocollins are associate d with desmoplakin and one or several of the plakophilins (PP1-3). Her e we describe a novel type of adhering junction comprising proteins of both AJs and desmosomes and the tight junction (TJ) plaque protein, Z O-1, in a newly established, liver-derived tumorigenic rat cell line ( RMEC-1). By immunofluorescence microscopy, cell-cell contacts are char acterized by mostly continuous-appearing lines which are usually resol ved by electron microscopy as extended arrays of closely spaced small plaque subunits. These plaque-covered regions are positive for plakogl obin, alpha- and beta-catenin, the arm-repeat protein p120, vinculin, desmoplakin and protein ZO-1. They are positive for E-cadherin in cult ures early on in passaging, but tend to turn negative for all known ca dherins in densely grown cultures. On immunoblotting SDS-PAGE-separate d proteins from dense-grown cell monolayers, ''pan-cadherin'' antibodi es have reacted with a band at similar to 140 kDa, identified as N-cad herin by peptide fingerprinting of the immunoprecipitated protein, whi ch for reasons not yet clear is modified or masked in immunolocalizati on experiments. The exact histological derivation of RMEC-1 cells is n ot known. However, the observations of several endothelial markers and the fact that all cells are rich in IFs containing vimentin and/or de smin, while only subpopulations also reveal Ifs containing CKs 8 and 1 8, is suggestive of a mesenchymal, probably endothelial origin. We dis cuss the molecular relationship of this novel type of extended junctio n with other types of adhering junctions.