The technique employing lactoperoxidase-cata-lyzed iodination for investigating protein distribution within erythrocyte stroma has been extended to examine the proteins of the surface membranes of mouse fibroblasts (L cells). The surface membranes were examined after four different conditions of iodination: (1) the intact cell, where only proteins exposed at the surface are susceptible to iodination by lactoperoxidase; (2) the intact cell in the absence of lactoperoxidase as a measure of nonspecific iodination; (3) the isolated surface membrane; and (4) the partially denatured surface membrane, where, presumably, most proteins are accessible. The surface membranes were solubilized in sodium dodecyl sulfate and the protein subunits were examined by disc gel electrophoresis. The results indicate that one group of polypeptides with a molecular weight of approximately 230,000 is in an exposed position on the exterior of the L cell surface membrane. Smaller amounts of radioactive iodine are associated with polypeptides of molecular weight 46,000- 150,000 suggesting that a small portion of their chain is exposed. No radioactivity is associated with two polypeptides of large molecular weight (170,000 and 190,000), a polypeptide of molecular weight 39,500, and several smaller polypeptides of molecular weight range 15,000-32,000. All proteins are iodinated when surface membranes were partially denatured and subsequently iodinated. The fact that the degree of iodination increases manyfold in the isolated membranes supports the observation that only a few proteins are exposed on the surface of the intact L cell. This iodination technique was extended to the surface membranes of baby hamster kidney fibroblasts, BHK21/C13, before and after transformation by the Bryan strain of Rous sarcoma virus, Ci3/B4. Again the results indicate that in both cases a large molecular weight group of polypeptides is exposed on the outside of the surface membrane.
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