Expression of translation initiation factor IF2 is regulated during osteoblast differentiation

We isolated and characterized a cDNA for the N-terminal half of the eukaryo tic initiation of translation factor 2 (cIF2) during a screen of chicken os teoblast cDNAs. The apparent size of the message for this protein, similar to5.6 kb, is slightly larger in size than that for human IF2 (hIF2). There is a high degree of sequence similarity between the human and chicken N-ter minal portions of the protein that extends to the encoding nucleotide seque nce. The tissue specific expression pattern for clF2 and hIF2 are similar, being moderately abundant in brain, liver, and skeletal muscle, and detecta ble in kidney, chondrocytes, and freshly isolated osteoblasts. The ratio of message for clF2 to that of beta -actin was 0.10 and 0.18 for liver and br ain. Message levels peak in osteoblasts between 8 and 12 days of culture, c oinciding with high levels of matrix protein synthesis. At peak expression, the ratio of cIF2:beta -actin for 8 day osteoblasts was 0.76. Treatment of osteoblast cultures with cycloheximide markedly reduces the level of clF2 message indicating that novel protein synthesis is required for its express ion. Hybridization of RNA samples from either chicken osteoblasts or a huma n osteoblast cell line with a probe for a subunit of human eukaryotic initi ation of translation factor 2 (eIF2 alpha), the housekeeping initiation fac tor, indicates that levels of eIF2 remain low. With hIF2, clF2 represents t he only other vertebrate homolog of IF2 for which a major portion of the co ding sequence has been identified. This is the first report of regulated ex pression for a eukaryotic IF2 and is the first demonstration of its abundan ce in osteoblasts. J. Cell. Biochem. 81:700-714, 2001. (C) 2001 Wiley-Liss, Inc.