High molecular weight forms of human placental lactogen: Synthesis in vitro and binding to membrane receptors for lactogenic hormones

Translation in wheat germ extracts of poly(A)-containing RNA isolated from human term placentas resulted in the synthesis of immunoreactive forms of human placental lactogen (hPL) capable of specific binding to lactogenic receptors. The minor component coelectrophoresed on sodium dodecyl sulfate-polyacrylamide gels with authentic hPL while the major component migrated with an apparent molecular weight about 3000 larger. In addition to this precursor-like molecule, even higher molecular weight forms of hPL were observed under certain conditions: (i) when the cell-free translation products were purified by precipitation with anti-hPL serum followed by dissociation of the immunoprecipitate in guanidine hydrochloride and chromatography of the solubilized material on Sephadex G-150 in the same denaturing buffer, and (ii) when the cell-free reaction mixture was analyzed by direct chromatography on Sephadex G-150 in nondenaturing buffers. Under both sets of conditions 50–75% of the radioactivity was eluted in the column void volume, suggesting it had a molecular weight of 150,000 or more. When the high molecular weight translated product was analyzed by electrophoresis on sodium dodecyl sulfate-polyacrylamide gels, the radioactive components were identical to authentic hPL and the precursorlike form, suggesting the large forms are aggregates of the smaller forms. Both the very high molecular weight forms, composed primarily of the precursor-like molecule, and the less aggregated products bound to specific lactogenic hormone receptors in rat liver membrane preparations, although the larger forms exchanged less readily with unlabeled hPL than did the monomeric form of the hormone. The aggregated, receptor-bindable cell-free translation product may be similar to high molecular weight lactogens previously described in vivo.