Functional and structural characterization of the two .beta.1-adrenoceptor forms in turkey erythrocytes with molecular masses of 50 and 40 kilodaltons

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Abstract
We have previously described a specific protease in turkey erythrocytes that converts the larger 50-kDa (P50) form of the .beta.1-adrenoceptor to a smaller 40-kDa (P40) form [Jurss, R., Hekman, M., and Helmreich, E. J. M. (1985) Biochemistry 24, 3349-3354]. Further functional and structural characterization studies of the two forms are reported here. When purified P50 and P40 receptors were compared with respect to their relative capabilities to couple in lipid vesicles with pure stimulatory G-proteins (Gs-proteins) prepared from turkey erythrocytes or rabbit liver, a faster and larger activation of Gs-proteins was observed in response to l-isoproterenol and guanosine 5''-O-(3-thiotriphosphate) (GTP.gamma.S) with P40 than with P50 receptor. The kon values for Pp40 were 0.47 min-1 in the case of liver Gs and 0.22 min-1 in the case of erythrocyte Gs, whereas the corresponding values for P50 were 0.34 min-1 and 0.12 min-1, respectively. The binding properties of P50 and P40 forms of the receptor were not different, and desensitization of turkey erythrocytes on exposure to l-isoproterenol did not activate the protease. We furthermore ascertained that only the larger form with a molecular mass of 50 kDa carries the N-linked carbohydrates, which are removed on proteolytic conversion to the 40-kDa form and have either a triantennary or a tetraantennary nonfucosylated complex-type structure containing terminal sialyl residues.

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