Characterization of hepatic lactogen receptor. Subcellular distribution and characterization of N-linked carbohydrate chains

The types of carbohydrate chains present in a rat liver lactogenic hormone-binding receptor species with an M(r) of 82000, and in its hormone-binding subunits with M(r) values of 40000 and 35000, were characterized using carbohydrate-chain-cleaving enzymes and affinity cross-linking. The subcellular distribution of lactogenic hormone-binding species was studied in organelle-enriched fractions. The monomeric M(r)-40000 and M(r)-35000 species contain N-linked tri- or tetra-antennary complex and high-mannose chains respectively. The M(r)-82000 species exists in two forms, where the M(r)-40000 and M(r)-35000 subunits are each combined with unglycosylated and, with the technique used, unlabelled subunit(s). Studies with organelle-enriched fractions revealed that the M(r)-35000 species was found in an endoplasmic reticulum-enriched fraction. The M(r)-40000 species was the predominant monomeric binding species in Golgi/endosome- and plasma membrane-enriched fractions. It is suggested that the M(r)-35000 species is a precursor to the M(r)-40000 species. In lysosome/endosome- or lysosome-enriched fractions, a broad distribution in M(r) (35000-40000) was characteristic of the hormone-binding species. The M(r)-82000 species was only found in a Golgi/endosome-enriched fraction. Labelling of endosome lactogen receptor by injection of ¹²⁵I-labelled ovine prolactin in vivo and cross-linking yielded only the M(r)-40000 species. Thus, the M(r)-40000 and M(r)-35000 lactogenic hormone-binding species each appear to be combined with the unglycosylated receptor subunit(s) in the Golgi complex to form M(r)-82000 heterodimeric complexes.