Abstract
1,25-Dihydroxycholecalciferol very rapidly stimulates the incorporation of [4,5-3H]leucine into at least two proteins of the chick intestinal mucosal cells. The smaller of the two proteins has mol.wt. approx. 42000, very similar to actin. Other properties of this protein were investigated, including its solubility in salt solution and its behaviour on gel electrophoresis and isoelectric focusing, and in all cases the 1,25-dihydroxycholecalciferol-stimulated protein was indistinguishable from intestinal actin. It was also shown that the brush borders of mucosal cells contain both beta- and gamma-actin in approximately equal amounts and that both forms of the protein appear to be affected by the hormone. It is concluded therefore that one of the earliest actions of 1,25-dihydroxycholecalciferol is to stimulate the incorporation of leucine into beta- and gamma-actin of the mucosal cells or into two proteins very like them.