Partial purification of somatotropin receptors from pig liver: they arise from a single somatotropin receptor messenger RNA transcript.

Specific binding sites for porcine somatotropin (pST) have been identified in pig liver microsomal membranes. Little information, however, is available about the size and number of ST receptor (ST-R) forms present. Therefore, the present study was conducted to characterize ST-R in pig liver using two approaches. In the first set of experiments, cross-linking of [125I]bST (bovine ST) to microsomal membranes, followed by gel electrophoresis under reducing conditions, revealed the presence of a predominant protein of 107 kDa and four other proteins of 71, 52, 40, and 26 kDa. In a second set of experiments, ST-R were partially purified using affinity chromatography. Binding studies indicated that there was an approximately 1,800-fold purification compared to liver homogenate. Two specific proteins of 107 and 40 kDa were detected after crosslinking of [125I]bST to partially purified ST-R. Northern blot analysis revealed that these proteins arise by posttranslational modification of a single 4.2-kilobase somatotropin receptor messenger RNA transcript. Although the present study indicates that several forms of ST-R are present in pig liver, it is not clear what physiological role these different ST-R play in mediating the hepatic effects of pST. It is evident, however, that the smaller proteins are generated from the 107-kDa protein, which is the predominant isoform present in liver microsomal membranes.