Specific, covalent binding of an azidoretinoid to cellular retinoic acid-binding protein

Photoaffinity labeling of plasma proteins

Photoaffinity labeling is a powerful technique for identifying a target protein. A high degree of labeling specificity can be achieved with this method in comparison to chemical labeling. Human serum albumin (HSA) and α₁-acid glycoprotein (AGP) are two plasma proteins that bind a variety of endogenous and exogenous substances. The ligand binding mechanism of these two proteins is complex. Fatty acids, which are known to be transported in plasma by HSA, cause conformational changes and participate in allosteric ligand binding to HSA. HSA undergoes an N-B transition, a conformational change at alkaline pH, that has been reported to result in increased ligand binding. Attempts have been made to investigate the impact of fatty acids and the N-B transition on ligand binding in HSA using ketoprofen and flunitrazepam as photolabeling agents. Meanwhile, plasma AGP is a mixture of genetic variants of the protein. The photolabeling of AGP with flunitrazepam has been utilized to shed light on the topology of the protein ligand binding site. Furthermore, a review of photoaffinity labeling performed on other major plasma proteins will also be discussed. Using a photoreactive natural ligand as a photolabeling agent to identify target protein in the plasma would reduce non-specific labeling.

Photoaffinity labeling of retinoic acid-binding proteins

Retinoid-binding proteins are essential mediators of vitamin A function in vertebrate organisms. They solubilize and stabilize retinoids, and they direct the intercellular and intracellular trafficking, transport, and metabolic function of vitamin A compounds in vision and in growth and development. Although many soluble retinoid-binding proteins and receptors have been purified and extensively characterized, relatively few membrane-associated enzymes and other proteins that interact with retinoids have been isolated and studied, due primarily to their inherent instabilities during purification. In an effort to identify and purify previously uncharacterized retinoid-binding proteins, it is shown that radioactively labeled all-trans-retinoic acid can be used as a photoaffinity labeling reagent to specifically tag two known retinoic acid-binding proteins, cellular retinoic acid-binding protein and albumin, in complex mixtures of cytosolic proteins. Additionally, a number of other soluble and membrane-associated proteins that bind all-trans-[11,12-3H]retinoic acid with high specificity are labeled utilizing the same photoaffinity techniques. Most of these labeled proteins have molecular weights that do not correspond to any known retinoid-binding proteins. Thus, photoaffinity labeling with all-trans-retinoic acid and related photoactivatable retinoids is a method that should prove extremely useful in the identification and purification of novel soluble and membrane-associated retinoid-binding proteins from ocular and nonocular tissues.

Reduced expression of PLC-gamma during the differentiation of mouse F9 teratocarcinoma cells

Expression of phosphoinositide-specific phospholipase C (PLC)-gamma during the retinoic acid (RA)-induced differentiation of F9 teratocarcinoma stem cells was investigated. PLC-gamma activity and PLC-gamma protein content were decreased during the differentiation of F9 teratocarcinoma stem cells induced by a combination (RACT) of RA, dibutyryl cyclic AMP (C) and theophylline (T). However, PLC-gamma activity and protein levels in RA- or C- and T-treated F9 cells remained the same as that of the F9 stem cells. Northern analysis of the 5.4-kb PLC-gamma transcript in differentiated F9 cells by RACT revealed that RACT decreased the PLC-gamma gene expression. These results suggest that the expression of PLC-gamma is negatively controlled by RACT treatment in the differentiation of F9 teratocarcinoma stem cells.