Retinoid-binding proteins are phosphorylated in vitro by soluble Ca+2- and phosphatidylserine-dependent protein kinase from mouse brain.

Cellular retinol-binding protein (cRBP) and cellular retinoic acid-binding protein (cRABP) were purified from calf liver and uterus, respectively. Soluble Ca+2- phosphatidylserine-dependent protein kinase-C (PK-C) derived from mouse brain was capable of phosphorylating both of these apoproteins in vitro as determined by the phosphocellulose binding assay. The Km value was determined to be 6.2 microM for apo-cRBP and 5.1 microM for apo-cRABP. In contrast, the Km value for the histone III-S fraction was estimated to be 10.8 microM; the Km values for ATP in the presence of apo-cRBP and apo-cRABP were 12.4 microM and 2.6 microM, respectively. Specificity of phosphorylation of the retinoid-binding proteins was confirmed by polyacrylamide gel electrophoresis and subsequent autoradiography of the assay mixture as well as by a concentration-dependent, Ca+2, and phosphatidylserine sensitivity of the phosphorylation of both apo-cRBP and apo-cRABP. Inhibition of PK-C activity by holo-cRBP and holo-cRABP was also observed. Thus, phosphorylation of both of the retinoid-binding proteins may play an important modulating role in i) the ability of retinoids to function as antipromoters in chemically-induced tumorigenesis and ii) the control of physiological aspects of retinoid action in normal and retrodifferentiated cells.