Biogenesis of the somatogenic receptor in rat liver

Certain structural characteristics, in particular the type of oligosaccharide chains associated with the rat liver somatogenic (GH) receptors, were studied in different isolated organelles involved in receptor biosynthesis, maturation, and binding, with the use of ligand-affinity cross-linking, incubation with various oligosaccharide chain-cleaving enzymes, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In an endoplasmic reticulum-enriched fraction, a somatogenic receptor with M(r) 33,000, after correction for bound ligand (assuming a 1:1 binding ratio of ligand to receptor) was found to contain N-linked high mannose oligosaccharide chain(s). In an intermediate density fraction, enriched in cis-Golgi, a major receptor of M(r) 43,000 was found to contain N-linked complex type of oligosaccharide chains. In a low density membrane fraction, containing trans-Golgi complex membranes and endocytic vesicles, three receptors of M(r) 95,000, 55,000, and 43,000 were found. These three receptors contain N-linked complex-type oligosaccharide chains. Neuraminidase treatment resulted in a decrease of the M(r) 95,000 and 43,000 receptors to M(r) 81,000 and 39,000, respectively. Two specific somatogenic receptors of M(r) 95,000 and 43,000 containing N-linked complex type of oligosaccharides were found in an isolated plasma membrane-enriched fraction. When isolated hepatocytes were analyzed, the M(r) 95,000 receptor was found to be the major labeled species. Two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis (first dimension nonreducing and the second dimension reducing conditions), showed that the M(r) 43,000 receptor is contained within the M(r) 95,000 receptor. The data suggest that the M(r) 33,000 receptor found in endoplasmic reticulum constitutes a precursor to the M(r) 43,000 receptor and that the M(r) 43,000 receptor is complexed with an unknown subunit during transport through the Golgi complex to form an M(r) 95,000 receptor present on the cell surface.