Expression of Variant Forms of Proopiomelanocortin, the Common Precursor to Corticotropin and Beta-lipotropin in the Rat Pars Intermedia

Proopiomelanocortin, the common glycoprotein precursor to adrenocorticotropin (ACTH) and beta-lipotropin (beta-LPH), is the most abundant protein synthesized in rat neurointermediate lobes. It represents 30% of the total amount of radioactive proteins obtained after a 1-h pulse incubation with [3H]phenylalanine. Several forms of this protein can be separated by a high-resolution two-dimensional gel electrophoresis technique. The three most abundant species which can be reproducibly characterized by their apparent molecular weights (Mr) and isoelectric points (pI) were called form I (Mr 34 000; pI 8.2), form II (Mr 36 000; pI 8.2), and form III (Mr 35 000; pI 7.3). Additional minor forms, representing together approximately 30% of the total forms I, II, and III combined, are also observed. They have very close molecular weights but differ by their isoelectric points. When glycosylation is prevented by tunicamycin, forms I and II are replaced by a new molecule with the same pI of 8.2 but a slightly lower Mr (32 000). This form is referred to as form T1. Similarly, form III is replaced by form T2 (Mr 33 000; pI 7.3). Forms T1 and T2 are supposed to be nonglycoslyated peptides. They were further characterized by microsequencing and peptide mapping. They both have the same N-terminal amino acid sequence with leucine residues in positions 3 and 11, and they both contain identical [3H]phenylalanine-labeled tryptic fragments, two of them corresponding to the sequences 1-8 of ACTH and 61-69 of beta-LPH. However, a limited digestion with the Staphylococcus aureus (V8 strain) protease generates a collection of peptides different for each form. These results suggest the presence of at least two different gene products corresponding to the major forms of proopiomelanocortin in the rat pars intermedia.