Characterization of cell surface changes occurring upon transformation and understanding of the biochemical mechanisms underlying these alterations are the win aims of the research project. Investigation has focused on membranes of chick embryo fibroblasts (CEFs) transformed by Rous sarcoma virus. The disulfide-bound subunit structure of the transformation-sensitive major surface protein LETS (MW 240,000) has been determined. Less than 5% is present as monomer, 25-50% as dimer, and the remainder as oligomer of 1 x 10 to the 6th power MW or greater. The possible relationship between the stability of LETS on the cell surface and its subunit structure is being examined. Further studies are directed at the use of reversible cross-linking reagents, e.g. 3,3'dithiobispropionimidate, and immune precipitation to analyze protein-protein interactions. Particular attention is being directed at interactions involving transformation-sensitive surface proteins and cytoskeletal components. We have found that the phorbol esters induce normal CEFs to reversibly form a partial phenocopy of an RSV-transformed cell. Among surface changes, LETS is decreased by 80%. Structure-activity comparisons indicate close correspondence between the potencies of phorbol derivatives for LETS loss in CEFs and their biological activities in mice. Current studies are directed at understanding the mechanism of phorbol diester induced LETS loss and the identification of the putative phorbol receptor in CEFs. BIBLIOGRAPHIC REFERENCES: Driedger, P.E. and Blumberg, P.M.: Effects of phorbol myristate acetate and related derivatives on chick fibroblasts. Fed. Proc. 36: 701a, 1977. Rossow, P.W., McConnell, M.R. and Blumberg, P.M.: The subunit structure of LETS protein on the surface of chick embryo fibroblasts. Fed. Proc. 36: 358a, 1977.