Redox activations of serine/threonine kinases represent alternate pathways in which vitamin A plays a crucial co-factor role. Vitamin A binds the zinc finger domain of c-Raf with nanomolar affinity. The retinoidbinding site has been mapped within this structure by scanning mutagenesis. The deduced contact sites were found anchored on Phe-8, counting from the 1st conserved histidine of the zinc finger. These sites agreed with contact amino acids identified by computational docking. The boundaries of a related binding pocket were identified by mutagenesis and partially confirmed by docking trials in the protein kinase C-␣ C1A zinc finger. They comprised Phe-7, Phe-8, and Trp-22. This trio was absent from the ␣C1B domain, explaining why the latter did not bind retinol. Reconfiguring at a minimum the two corresponding amino acids of ␣C1B, Thr-7 and Tyr-22, to conform to ␣C1A converted this domain to a binder. Deletion of the predicted retinoid-binding site in the full-length molecule created a mutant c-Raf that was deficient in retinol-dependent redox activation but fully responsive to epidermal growth factor. Our findings indicate that ligation of retinol to a specific site embedded in the regulatory domain is an important feature of c-Raf regulation in the redox pathway.The history of vitamin A research contains a medley of observations concerning widespread physiological roles of retinoids other than the well known functions of retinoic acid in transcription and retinaldehyde in vision (reviewed in Ref. 1). Most convincing for the non-nuclear functions of vitamin A are arguments pointing to the evolution of an elaborate retinoid biochemistry and biology in eukaryotic organisms (2), predating by far the advent of retinoid retinoic acid receptors and retinoid X receptors and the conservation of the vitamin A metabolites, along with the requisite enzymes, from insects to man. Furthermore, essentially all nucleated cells of higher vertebrates store vitamin A in the form of retinyl esters for ready retrieval and conversion to a variety of metabolites. Because these retinoid products, more often than not, exclude retinoic acid, the question arises as to their purpose.The multitude of defects caused by nutritional vitamin A deficiency, not completely reversible by retinoic acid and ranging from multiple developmental abnormalities (3, 4), to immune defects (5-8), and to male sterility (9, 10), was not explainable by a single non-nuclear target. In fact, multiple molecular targets emerged when the serine/threonine kinases were found by us to harbor high affinity retinoidbinding sites (11). These were encoded within the cysteinerich domains of several PKC 1 isoforms and c-Raf and overlapped with known structures intimately involved with kinase regulation. This is where lipid second messengers bind and activate the conventional and novel PKC isoforms (12-14) and where, in c-Raf, a crucial half-site is located for recognition of the activating GTP/Ras protein (15-17). Nevertheless, the purpose of retinoid-binding sites rema...