Control of Type I Collagen Systhesis: Evidence for Pretranslational Coordination of Proα1 (I) and Proα2 (I) Chain Synthesis in Embryonic Chick Bone

Abstract

The normal type I collagen molecule contains 2 .alpha.1 (I) chains and 1 .alpha.2 (I) chain. In embryonic chick calvaria, the 2 chains are synthesized in a 2:1 ratio, and total polysomes from this tissue contain twice as much mRNA for pro.alpha.1 (I) as for pro.alpha.2 (I)9. To further investigate the mechanism by which synthesis may be coordinated, RNA isolated from various cell fractions of embryonic chick calvaria was translated in a rabbit reticulocyte lysate cell-free system. The procollagen chain products were separated by gel-electrophoresis and densitometrically quantitated from autoradiograms of the gels. Total cellular RNA, total cytoplasmic RNA, and polysomal RNA each directed the synthesis of pro.alpha.1 (I) and pro.alpha.2 (I) in a proportion of 2:1, whereas no procollagen mRNA activity was found in nonpolysomal cytoplasmic RNA. In the chick bone cells, all compartments apparently contain twice as much pro.alpha.1 (I) mRNA as pro.alpha.2 (I) mRNA, and that virtually all procollagen mRNA in the cytoplasm is polysome-bound. The coordination of procollagen chain synthesis thus presumably occurs at a pretranslational level, though differential rates of formation and/or degradation of the 2 mRNA.