Expression of variant forms of proopiomelanocortin, the common precursor to corticotropin and .beta.-lipotropin in the rat pars intermedia

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Abstract
Proopiomelanocortin, the common glycoprotein precursor to 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 high-resolution 2-dimensional gel electrophoresis technique. The 3 most abundant species which can be reproducibly characterized by their apparent MW and isoelectric points (pI) were called form I (MW 34,000; pI 8.2), form II (MW 36,000; pI 8.2) and form III (MW 35,000; pI 7.3). Additional minor forms, representing together .apprx. 30% of the total of forms I, II and III combined, are also observed. They have very close MW but differ by their pI. 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 MW (32,000). This form is referred to as form T1. Form III is replaced by form T2 (MW 33,000; pI 7.3). Forms T1 and T2 are supposed to be nonglycosylated 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, 2 of them corresponding to the sequences 1-8 of ACTH and 61-69 of .beta.-LPH. A limited digestion with the Staphylococcus aureus (V8 strain) protease generates a collection of peptides different for each form. The presence of at least 2 different gene products corresponding to the major forms of proopiomelanocortin in the rat pars intermedia is suggested.