Conservation of function between mammalian and plant steroid 5 alpha-reductases

Arabidopsis det2 mutants are small dark-green dwarfs displaying pleiotropic defects in light-regulated development during multiple stages of the plant life cycle. The DET2 gene encodes a protein that shares approximate to 40% sequence identity with mammalian steroid 5 alpha-reductases and is implicated in the synthesis of a class of plant steroids, the brassinosteroids. Here we show that the DET2 protein, when expressed in human embryonic kidney 293 cells, catalyzes the 5 alpha-reduction of several animal steroid substrates and has similar kinetic properties to the mammalian steroid 5 alpha-reductase enzymes. Moreover, human steroid 5 alpha-reductases expressed in det2 mutant plants can substitute for DET2 in brassinosteroid biosynthesis. These data indicate that DETZ is an ortholog of the mammalian steroid 5 alpha-reductases and provide further evidence that brassinosteroids play an essential role in light-regulated plant development. The structural and functional conservation between DET and human steroid 5 alpha-reductases raise interesting issues concerning the evolutionary origin of the steroid hormone signaling system.