LOCALIZATION OF CRANIN (DYSTROGLYCAN) AT SITES OF CELL-MATRIX AND CELL-CELL CONTACT - RECRUITMENT TO FOCAL ADHESIONS IS DEPENDENT UPON EXTRACELLULAR LIGANDS

We report that cranin (dystroglycan) can become recruited to focal adh esions of cultured rat REF 52 fibroblasts and human aortic smooth musc le cells. Within mature focal adhesions, cranin was present within the plaque region defined by beta 1 integrin, Vinculin and phosphotyrosin e staining, but occupied a larger domain corresponding to the terminal segments of stress fibers that was more precisely co-extensive with t he cytoskeletal proteins alpha-actinin, utrophin and aciculin. When RE F 52 fibroblasts were plated on different substrata in the absence of protein synthesis and secretion in serum-free medium, focal clusters o f cranin readily formed within 2 hours on matrix proteins that bind cr anin directly (laminin or agrin) which were maintained as the focal ad hesions became mature, In contrast, cranin failed to become targeted t o cell-substratum attachment sites, either at early or later times, wh en cells were plated on a variety of other substrata that elicit forma tion of focal adhesions but do not bind cranin directly (fibronectin, vitronectin, collagen type IV, or anti-beta 1 integrin antibody TS2/16 ). These data strongly suggest that targeting of cranin to focal adhes ions was dependent upon the presence of an extracellular ligand capabl e of binding cranin directly, However, some cultured nonmuscle cell li nes (e.g., human umbilical vein endothelial cells, NIH 3T3 and CHO cel ls) failed to localize cranin to focal adhesions, even when plated on laminin. Cranin was also enriched at cell-cell adherens-type junctions of human normal breast MCF-10 epithelial cells, and at growth cones o f E17 rat hippocampal axons. That cranin can become targeted to sites of cell-cell and cell-substratum contact in diverse cell types support s the hypothesis that cranin may be involved in mediating or regulatin g cell adhesion. The absence of muscle-specific and synapse-specific p roteins within fibroblasts and epithelial cells provides a different c ontext for thinking about cranin (dystroglycan) that may aid in discer ning general principles of its structure and function.