CONSERVATION OF FUNCTION BETWEEN MAMMALIAN AND PLANT STEROID 5-ALPHA-REDUCTASES

Arabidopsis det2 mutants are small dark-green dwarfs displaying pleiot ropic defects in light-regulated development during multiple stages of the plant life cycle. The DET2 gene encodes a protein that shares app roximate to 40% sequence identity with mammalian steroid 5 alpha-reduc tases and is implicated in the synthesis of a class of plant steroids, the brassinosteroids. Here we show that the DET2 protein, when expres sed in human embryonic kidney 293 cells, catalyzes the 5 alpha-reducti on of several animal steroid substrates and has similar kinetic proper ties to the mammalian steroid 5 alpha-reductase enzymes. Moreover, hum an steroid 5 alpha-reductases expressed in det2 mutant plants can subs titute for DET2 in brassinosteroid biosynthesis. These data indicate t hat DETZ is an ortholog of the mammalian steroid 5 alpha-reductases an d provide further evidence that brassinosteroids play an essential rol e in light-regulated plant development. The structural and functional conservation between DET and human steroid 5 alpha-reductases raise in teresting issues concerning the evolutionary origin of the steroid hor mone signaling system.