Quantification and characterization of regulin, a Mr-230,000 highly elongated protein of rabbit reticulocytes

Procedures are described by which regulin in rabbit reticulocytes was quantified and isolated in relatively large amounts. In these cells the protein occurs at a ratio of about 1.1-1.6 regulin monomers/spectrin tetramer, corresponding to 80,000-100,000 molecules of Mr-230,000 regulin/cell. Erythrocytes contain less than 12% of the amount of regulin in reticulocytes and the protein has not been detected in non-erythroid cells. Regulin was found primarily in the cytosolic fraction of lysed reticulocytes. It appears to be unusually sensitive to proteolysis by Ca2+-activated thiol proteases. Isolation of Mr-230,000 undegraded regulin was accomplished by the use of protease inhibitors including N-ethylmaleimide. A striking characteristic of regulin is its tendency to aggregate in neutral solution of low ionic strength. Physical studies of the isolated protein indicate that it has a highly elongated form in solution. The protein has no known enzymatic activity but was shown previously to interact with and increase the enzymatic activity of a protein phosphatase. The properties of regulin suggest that it may have a structural function but it appears to be physically and immunologically distinct from known proteins. It is suggested that regulin may contribute to a gel matrix within the cytoplasm of reticulocytes.