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The molecular properties of cold-insoluble globulin have been investigated by velocity centrifugation, circular dichroism, and fluorescence at neutral and alkaline pH. The stability of the protein to thermal and guanidine hydrochloride has been evaluated under both conditions. The close parallelism between the properties of cold-insoluble globulin and those of the cell surface protein (fibronectin) serve to establish the essential identity of the structures of the two proteins derived from different sources. It is suggested that the cold-insoluble globulin is composed of several domains connected by flexible polypeptide segments. The large increase in the frictional ratio observed between pH 7.0 and 11.0 can be explained by an expansion of the flexible segments without significant change in the domains. These domains are stable to about 55 degrees C at pH 7.0 but only to about 40 degrees C at pH 11.0.