Identity of p36K phosphorylated upon Rous sarcoma virus transformation with a protein purified from brush borders; calcium‐dependent binding to non‐erythroid spectrin and F‐actin

Membrane vesicles derived from the apical side of procine intestinal epithelial cells retain, after demembranation in the presence of calcium, two major proteins (I, II) which are released by the addition of calcium chelators. We have purified and characterized these two calcium‐binding proteins. Protein I has a mol. wt. of 85 000 and contains two copies of a 36‐K subunit and an additional 10‐K subunit. It binds in a calcium‐dependent manner to F‐actin as well as to non‐erythroid spectrin. Immunofluorescence microscopy reveals protein I‐related antigens in the terminal web of the intestinal cell and in a submembraneous cortical layer in various tissue culture cells. Biochemical and immunological results document that the 36‐K subunit of protein I is identical with the cellular p36K recognized as a major substrate for tyrosine phosphorylation by the sarc gene kinase in Rous sarcoma virus‐transformed cells. The biochemical properties of protein I agree with its location seen in immunofluorescence microscopy and cell fractionation and suggest that the actin‐spectrin network in the cortical layer may be affected by virus transformation.