Inhibition mechanism of human sterol O-acyltransferase 1 by competitive inhibitor

Abstract

24Sterol O-acyltransferase 1 (SOAT1) is an endoplasmic reticulum (ER) resident, multi-25 transmembrane enzyme that belongs to the membrane-bound O-acyltransferase (MBOAT) 26 family 1 . It catalyzes the esterification of cholesterol to generate cholesteryl esters for cholesterol 27 storage 2 . SOAT1 is a target to treat several human diseases 3 . However, its structure and 28 mechanism remain elusive since its discovery. Here, we report the structure of human SOAT1 29 (hSOAT1) determined by cryo-EM. hSOAT1 is a tetramer consisted of a dimer of dimer. The 30 structure of hSOAT1 dimer at 3.5 Å resolution reveals that the small molecule inhibitor CI-976 31 binds inside the catalytic chamber and blocks the accessibility of the active site residues H460, 32 N421 and W420. Our results pave the way for future mechanistic study and rational drug design 33 of SOAT1 and other mammalian MBOAT family members. 34 35 36 37 38 39 40 41 42 43 44 45 46 Main 47Cholesterol is an essential lipid molecule in the cell membranes of all vertebrate. It is important 48 for maintaining the fluidity and integrity of the membrane and is the precursor for the 49 biosynthesis of other crucial endogenous molecules, such as steroid hormones and bile acids. In 50 addition, cholesterol can modulate the activity of many membrane proteins such as GPCR 4 and 51 ion channels 5 . The concentration of cellular free cholesterol is highly regulated 2 . Excessive 52 intracellular cholesterol may form cholesteryl esters, which are catalyzed by the enzyme, sterol 53 O-acyltransferase (SOAT), also called acyl-coenzyme A: cholesterol acyltransferase (ACAT). 54 SOAT catalyzes the reaction between long chain fatty acyl-CoA and intracellular cholesterol to 55 form the more hydrophobic cholesteryl ester, which is then stored in lipid droplets within the cell 56 or transported in secreted lipoprotein particles to other tissues that need cholesterol. In addition 57 to cholesterol, SOAT can use multiple sterols as substrates and activators 3 . Because of its 58 functional importance, SOAT1 is a potential drug target for Alzheimer's disease 6 , 59 atherosclerosis 7 and several types of cancers 8-11 . 60 Previous studies have shown that SOAT1 is an ER-localized multi-transmembrane protein that is 61 evolutionary conserved from yeast to humans 12 . There are two SOAT enzymes in mammals: 62 SOAT1 and SOAT2, which have a protein sequence identity of 48% in human (258 out of 537 63 residues aligned). SOAT1 is ubiquitously expressed in many types of cells 13 ; while SOAT2 is 64 mainly expressed in the small intestine and liver 14 . Due to the pathophysiological importane of 65 SOAT, many SOAT inhibitors of various strutural types have been made. Among them, the 66 small molecule SOAT inhibitor CI-976 exhibit competitive inhibition against fatty acyl-CoA 15 . 67 SOAT is the founding member of the membrane-bound O-acyltransferase (MBOAT) enzyme 68 family, which transfers the acyl chain onto various substrates, including lipids, peptides and 69 small proteins. There are 11 ...