Vitamin D-dependent phosphorylation of an intestinal protein

The complete pattern of response of target tissues to steroid hormones is still unclear although it seems that the initial biochemical events involve the transcriptional control of protein synthesis. A still outstanding problem concerns the role of these hormone-dependent proteins in the physiological response. Synthesis of several intestinal proteins is known to be stimulated by 1,25-dihydroxyvitamin D (1,25-(OH)2D)' at least one, calcium-binding protein (CaBP) by a transcriptional event. Steroid hormones also stimulate protein phosphorylation and we show here that 1,25-(OH)2D acting in vivo stimulates the ATP phosphorylation of a chick intestinal brush-border membrane protein of molecular weight (MW) 84,000. The phosphate is attached to the protein through a serine or threonine residue. The reaction is stimulated by Ca2+ but does not require Mg2+, and kinetic studies showed that 1,25-(OH)2D acts on this process by increasing the number of sites rather than the affinity of the substrate for the phosphate group. This phosphorylated protein has similar characteristics to those of the intestinal membrane protein whose synthesis is stimulated by 1,25-(OH)2D.