Biosynthesis of osteogenic growth peptide via alternative translational initiation at AUG₈₅ of histone H4 mRNA

The osteogenic growth peptide (OGP) is an extracellular mitogen identical to the histone H4 (H4) COOH-terminal residues 90-103, which regulates osteogenesis and hematopoiesis. By Northern analysis, OGP mRNA is indistinguishable from H4 mRNA. Indeed, cells transfected with a construct encoding [His¹⁰²]H4 secreted the corresponding [His¹³]OGP. These results suggest production of OGP from H4 genes. Cells transfected with H4-chloramphenicol acetyltransferase (CAT) fusion genes expressed both "long" and "short" CAT proteins. The short CAT was retained following an ATG → TTG mutation of the H4 ATG initiation codon, but not following mutation of the in-frame internal ATG₈₅ codon, which, unlike ATG₁, resides within a perfect context for translational initiation. These results suggest that a PreOGP is translated starting at AUG₈₅. The translational initiation at AUG₈₅ could be inhibited by optimizing the nucleotide sequence surrounding ATG₁ to maximally support upstream translational initiation, thus implicating leaky ribosomal scanning in usage of the internal AUG. Conversion of the predicted PreOGP to OGP was shown in a cell lysate system using synthetic [His¹⁰²]H4-(85-103) as substrate. Together, our results demonstrate that H4 gene expression diverges at the translational level into the simultaneous parallel production of both H4, a nuclear structural protein, and OGP, an extracellular regulatory peptide.