Introduction

Figure 1. This image shows the location of the DGAT protein within the Endoplasmic Reticulum Membrane

DGAT, or Diacylglycerol Acyltransferase, is a polytopic endoplasmic reticulum membrane protein embedded within the membrane of the ER. DGAT is highly expressed in epithelial cells of the small intenstine of homo sapiens. It can also be found in the liver, where it helps synthesize fats for storage, and the female mammary glands, where it produces fat in the milk. ^[1]

DGAT was originally discovered by its homology to Acyl-CoA cholesterol acyltransferases (ACAT) 1 and 2. The structure, catalytic mechanism of diacylglycerol acyltransferase, and how DGAT interacts with CoA was discovered using a Cryo-EM. The Cryo-EM map revealed that DGAT forms a dimer, with each subunit containing nine transmembrane helices. The N and C terminals of each helix are located on the cytosolic and luminal sides of the endoplasmic reticulum membrane respectively (Figure 1). ^[2]

DGAT makes triglycerides from a diglyceride in plasma (Figure 2). In order to do this, DGAT uses two substrates: a fatty acyl-CoA and a diacylglycerol substrate. The basic mechanism consists of a lone pair on a hydroxyl group of glycerol attacking the carbon of the thioester bond of CoA. This results in the breakage of the thioester bond, and the attached acyl group attaches to the glycerol, creating a triglyceride.

Figure 2. General schematic of DGAT's function.

Structure

DGAT consists of two domains, one cytoplasmic and one luminal. The cytoplasmic domain interacts with the interior of the cell and relays signals. The luminal domain senses misfolded proteins. The structure of DGAT consists of two protein chains, one ligand, two polymers, eighteen and zero beta sheets. The transmembrane helices form a large central cavity within the membrane that opens to the bilayer via a wide lateral gate. Through openings on the cytosolic and luminal sides of DGAT, this central cavity is also accessible. The majority of the transmembrane helices present within the structure also form a concave-shaped ridge on either side of the membrane. These aspects of the domain structure are deemed as the 'MBOAT core'. Within this core, a tunnel-like region, similar to a binding pocket, is also present. Access to the active site of DGAT by substrates is done through the lateral gate, which lies on the ER lumen side, within the membrane. This tunnel-like region is referred to as the . ^[1]

Figure 3. Shows the location of the lateral gate as the entrance to the cytosolic tunnel via the ER lumen side of the membrane.

The DGAT dimer structure is formed primarily through many hydrogen-bonding interactions between the first 20 resolved residues (His69-Gly87). Hydrophobic interactions of the transmembrane helix region (Phe82-Ile98) with the other monomer also support the dimer structure formation. Additionally, there are four phospholipids present at the dimer interface that have been thought to contribute to the interactions between DGAT monomers. ^[2]

Tunnels

DGAT consists of 3 tunnels, a cytosolic tunnel, an ER-luminal funnel, and a membrane-embedded lateral gate. The cytosolic tunnel is the site of acyl-CoA binding, with the CoA group pointing at the cytosolic face and its acyl chain pointing towards the endoplasmic reticulum lumen. DAG then enters via the lateral gate on the luminal side via the lateral gate where it can then access the active site. The resulting product can then be released to either side of the membrane. ^[1].