Acyl-CoA:monoacylglycerol acyltransferases (MGATs) and diacylglycerol acyltransferases (DGATs) catalyze the two consecutive steps in the synthesis of triacylglycerol, a key process required for dietary fat absorption into the enterocytes of the small intestine. In this report, we investigated the tendency of MGAT2 to form an enzyme complex with DGAT1 and DGAT2 in intact cells. We demonstrated that in addition to the 38-kDa monomer of the MGAT2 enzyme predicted by its peptide sequence, a 76-kDa moiety was detected in SDS-PAGE without reducing agent and heat inactivation. The 76-kDa MGAT2 moiety was greatly enhanced by treatment with a cross-linking reagent in intact cells. Additionally, the cross-linking reagent dose-dependently yielded a band corresponding to the tetramer (152 kDa) in SDS-PAGE, suggesting that the MGAT2 enzyme primarily functions as a homotetrameric protein and as a tetrameric protein. Likewise, DGAT1 also forms a homodimer under nondenaturing conditions. When co-expressed in COS-7 cells, MGAT2 heterodimerized with DGAT1 without treatment with a cross-linking reagent. MGAT2 also co-eluted with DGAT1 on a gel filtration column, suggesting that the two enzymes form a complex in intact cells. In contrast, MGAT2 did not heterodimerize with DGAT2 when co-expressed in COS-7 cells, despite high sequence homology between the two enzymes. Furthermore, systematic deletion analysis demonstrates that N-terminal amino acids 35-80 of DGAT1, but not a signal peptide at the N terminus of MGAT2, is required for the heterodimerization. Finally, co-expression of MGAT2 with DGAT1 significantly increased lipogenesis in COS-7 cells, indicating the functional importance of the dimerization.