Structural basis for catalysis and substrate specificity of human ACAT1

Acyl-coenzyme A: cholesterol acyltransferases (ACATs) catalyze acyl transfer from acyl-coenzyme A (CoA) to cholesterol to generate cholesteryl ester, which is the primary form for cellular storage and plasma transport of cholesterol. Because of their close relationship with cholesterol metabolism, ACATs represent potential drug target for the treatment of atherosclerosis and other cholesterol-related disorders. Here we present the cryo-EM structure of human ACAT1 at 3.3 angstrom resolution for dimer of dimers and 3.0 angstrom for a dimer. Each protomer consists of nine transmembrane segments that enclose a cytosolic (C) and a transmembrane (T) tunnel. The tunnels, each accommodating an elongated density, converge at the predicted catalytic site. Structure-guided mutational analyses suggest the cytosolic and lateral entry for acyl-CoA and cholesterol, respectively. Our structural, biochemical, and mass spectrometric characterizations reveal the catalytic mechanism and substrate preference for unsaturated acyl chain by ACAT1.