Catalyzes the formation of fatty acid-cholesterol esters, which are less soluble in membranes than cholesterol. Plays a role in lipoprotein assembly and dietary cholesterol absorption. Utilizes oleoyl-CoA ((9Z)-octadecenoyl-CoA) and linolenoyl-CoA ((9Z,12Z,15Z)-octadecatrienoyl-CoA) as substrates. May provide cholesteryl esters for lipoprotein secretion from hepatocytes and intestinal mucosa. a long-chain fatty acyl-CoA + a sterol = a sterol ester + CoA an acyl-CoA + cholesterol = a cholesterol ester + CoA (9Z)-octadecenoyl-CoA + cholesterol = cholesteryl (9Z-octadecenoate) + CoA (5Z,8Z,11Z,14Z,17Z)-eicosapentaenoyl-CoA + cholesterol = (5Z,8Z,11Z,14Z,17Z-eicosapentaenoyl)-cholesterol + CoA (9Z,12Z,15Z)-octadecatrienoyl-CoA + cholesterol = (9Z,12Z,15Z-octadecatrienoyl)-cholesterol + CoA (5Z,8Z,11Z,14Z)-eicosatetraenoyl-CoA + cholesterol = cholesteryl (5Z,8Z,11Z,14Z)-eicosatetraenoate + CoA May form homo- or heterodimers (By similarity). Interacts with INSIG1; the interaction is direct and promotes association with AMFR/gp78 (By similarity). Liver and intestine. Expression is six-fold greater in hepatocytes than in kupffer cells. Each protomer consists of 9 transmembrane segments, which enclose a cytosolic tunnel and a transmembrane tunnel that converge at the predicted catalytic site: acyl-CoA enters the active site through the cytosolic tunnel, whereas cholesterol enters from the side through the transmembrane tunnel. Polyubiquitinated by AMFR/gp78 at Cys-281, leading to its degradation when the lipid levels are low. Association with AMFR/gp78 is mediated via interaction with INSIG1. High concentration of cholesterol and fatty acid results in Cys-281 oxidation, preventing ubiquitination at the same site, resulting in protein stabilization. Oxidized at Cys-281: high concentration of cholesterol and fatty acid induce reactive oxygen species, which oxidizes Cys-281, preventing ubiquitination at the same site, and resulting in protein stabilization. Belongs to the membrane-bound acyltransferase family. Sterol o-acyltransferase subfamily.