Structure and proteolysis of the growth hormone receptor on rat hepatocytes

Abstract

125I-Labeled human growth hormone is isolated in high molecular weight (Mr) (300 000, 220 000, and 130 000) and low molecular weight complexes on rat hepatocytes after affinity labeling. The time-dependent formation of low molecular weight complexes occurred at the expense of the higher molecular weight species and was inhibited by low temperature or inhibitors of serine proteinases. Exposure to reducing conditions induced loss of Mr 300 000 and 220 000 species and augmented the amount of Mr 130 000 complexes. The molecular weight of growth hormone (22 000) suggests that binding had occurred with species of Mr 280 000, 200 000, and 100 000. Two-dimensional gel electrophoresis demonstrated that the 100 000-dalton receptor subunit is contained in both the 280 000- and 200 000-dalton species. Reduction of interchain disulfide bonds in the growth hormone receptor did not alter its elution from gel filtration columns, but intact, high molecular weight receptor constituents were separated from lower molecular weight degradation products. Digestion of affinity-labeled growth hormone.sbd.receptor complexes with neuraminidase increased the mobility of receptor constituents on sodium dodecyl sulfate.sbd.polyacrylamide gel electrophoresis. These observations show that the growth hormone receptor is degraded by hepatic serine proteinases to low molecular weight degradation products which can be separated from intact receptor by gel filtration. Intact hormone.sbd.receptor complexes are aggregates of 100 000-dalton sialoglycoprotein subunits held together by interchain disulfide bonds and by noncovalent forces.