Structure and function of retinoid-binding proteins.

Abstract

Biological processes which require compounds derived from vitamin A (retinol) include morphogenesis, spermatogenesis, the maintenance of epithelial tissue, and vision. For the last process the light-activated cis-trans isomerization of rhodopsin-bound retinaldehyde triggers a cascade of events which ultimately results in the hydrolysis of cyclic GMP. For morphogenesis, spermatogenesis, and the maintenance of epithelial tissue it is presumably retinoic acid that is the active metabolite. Both 9-cis and all-trans retinoic acid are transcriptional activators which exert their effects through binding to nuclear proteins that are members of the superfamily that includes the steroid hormone receptors. Various proteins mediate the transport and metabolism of the hydrophobic vitamin and consequently modulate the concentration of retinoic acid available to the nucleus. Both extracellular and intracellular retinoid-binding proteins have been identified in numerous cell types. In addition, the eye has retinoid binding proteins which are distinct for that organ: a cellular retinaldehyde-binding protein (CRalBP) and a interstitial/interphotoreceptor retinoid-binding protein (I-RBP).