Creation of a fully active, cytosolic form of human type I 3beta-hydroxysteroid dehydrogenase/isomerase by the deletion of a membrane-spanning domain.
Human 3beta-hydroxysteroid dehydrogenase/steroid Delta(5)-Delta(4)-isomerase (3beta-HSD/isomerase) is a bifunctional, single enzyme protein that is membrane-bound in the endoplasmic reticulum (microsomes) and mitochondria of cells in the placenta (type I) and in the adrenals and gonads (type II). Two membrane-binding domains (residues 72-89 and 283-310) have been predicted by analyses of hydrophobicity in the type I and II isoenzymes (90% regional homology). These putative membrane domains were deleted in the cDNA by PCR-based mutagenesis, and the two mutant enzymes were expressed by baculovirus in insect Sf9 cells. Differential centrifugation of the Sf9 cell homogenate containing the 283-310 deletion mutant revealed that 94% of the 3beta-HSD and isomerase activities were in the cell cytosol, 6% of the activities were in the microsomes, and no activity was in the mitochondria. This is the opposite of the subcellular distribution of the wild-type enzyme with 94% of the activities in the microsomes and mitochondria and only 6% activity in the cytosol. The organelle distribution of the 72-89 deletion mutant lies between these two extremes with 72% of the enzyme activity in the cytosol and 28% in the microsomes/mitochondria. The integrity of the subcellular organelle preparations was confirmed by electron microscopy. Western immunoblots confirmed the presence of the 283-310 deletion mutant enzyme and the absence of the wild-type enzyme in the insect cell cytosol. The unpurified, cytosolic 383-310 deletion mutant exhibited 3beta-HSD (22 nmol/min per mg) and isomerase (33 nmol/min per mg) specific activities that were comparable with those of the membrane-bound, wild-type enzyme. The isomerase reaction of the cytosolic 283-311 deletion mutant requires activation by NADH just like the isomerase of the microsomal or mitochondrial wild-type enzyme. In contrast, the 72-89 deletion mutant had low 3beta-HSD and isomerase specific activities that were only 12% of the wild-type levels. This innovative study identifies the 283-310 region as the critical membrane domain of 3beta-HSD/isomerase that can be deleted without compromising enzyme function. The shorter 72-89 region is also a membrane domain, but deletion of this NH(2)-terminal region markedly diminishes the enzyme activities. Purification of the active, cytosolic 283-310 deletion mutant will produce a valuable tool for crystallographic studies that may ultimately determine the tertiary/quaternary structure of this key steroidogenic enzyme.
Thomas
J L
JL
Department of Obstetrics and Gynecology, Washington University School of Medicine, St Louis, Missouri, USA.
Evans
B W
BW
Blanco
G
G
Mason
J I
JI
Strickler
R C
RC
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D015153:Blotting, Western; D002460:Cell Line; D003600:Cytosol / Q000201:enzymology*; D017931:DNA Primers; D006801:Humans; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008854:Microscopy, Electron; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D011375:Progesterone Reductase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017384:Sequence Deletion; D018411:Spodoptera / Q000648:ultrastructure; D013251:Steroid Isomerases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D013347:Subcellular Fractions / Q000201:enzymology
C021425:3 beta-hydroxysteroid oxidoreductase-delta(5) 3-ketosteroid isomerase; D017931:DNA Primers; D008565:Membrane Proteins; D009097:Multienzyme Complexes; D011994:Recombinant Proteins; D011375:Progesterone Reductase; D013251:Steroid Isomerases