The crystal structure of human microsomal triglyceride transfer protein

Abstract

Microsomal triglyceride transfer protein (MTP) plays an essential role in lipid metabolism, especially in the biogenesis of very low-density lipoproteins and chylomicrons via the transfer of neutral lipids and the assembly of apoB-containing lipoproteins. Our understanding of the molecular mechanisms of MTP has been hindered by a lack of structural information of this heterodimeric complex comprising an MTPα subunit and a protein disulfide isomerase (PDI) β-subunit. The structure of MTP presented here gives important insights into the potential mechanisms of action of this essential lipid transfer molecule, structure-based rationale for previously reported disease-causing mutations, and a means for rational drug design against cardiovascular disease and obesity. In contrast to the previously reported structure of lipovitellin, which has a funnel-like lipid-binding cavity, the lipid-binding site is encompassed in a β-sandwich formed by 2 β-sheets from the C-terminal domain of MTPα. The lipid-binding cavity of MTPα is large enough to accommodate a single lipid. PDI independently has a major role in oxidative protein folding in the endoplasmic reticulum. Comparison of the mechanism of MTPα binding by PDI with previously published structures gives insights into large protein substrate binding by PDI and suggests that the previous structures of human PDI represent the “substrate-bound” and “free” states rather than differences arising from redox state.

A point mutation decouples the lipid transfer activities of microsomal triglyceride transfer protein

“…The leucine to proline mutation is likely to disrupt helix 10 and the packing of the alpha-helical domain against the lipid-binding domain, thereby affecting lipid transfer activity indirectly. This is consistent with reported mutations in this region of the alpha-helical domain that cause abetalipoproteinemia, including L435H, Y528H and S590I (Reviewed in [30]), affecting lipid transfer activity indirectly by inducing conformational changes and/or destabilizing the structure [16,62,63]. Our immunoprecipitation data indicating that the L475P mutant protein fails to bind PDI also suggests that this mutation may be destabilizing the structure (Fig 5D).…”

“…The MTP M subunit has three major structural domains: an N-terminal half beta-barrel, a middle alpha-helical domain, and a C-terminal domain consisting of two beta-sheets and two alpha-helices that encompasses the lipid-binding site [16]. The amino acid sequence of the zebrafish Mtp M subunit is 54% identical to that of the human protein, while the PDI P subunits are~75% identical.…”

“…MTP is a heterodimer of the large M or MTPα subunit (~97 kDa, encoded by the MTTP gene) and the smaller P or MTPβ subunit, protein disulfide isomerase (PDI;~58 kDa subunit) [15,16]. Vertebrate MTP can bind and transfer triacylglycerol, diacylglycerol, phospholipid, cholesteryl ester, ceramide, and sphingomyelin between vesicles in vitro [17][18][19][20][21][22].…”

Dynamic plasticity of the lipid antigen-binding site of CD1d is crucially favoured by acidic pH and helper proteins

“…This assembly requires assistance from the ER-resident microsomal triglyceride transfer protein (MTP) which also facilitates loading of endogenous lipids onto CD1d in the ER 5,17 . Very recently, the crystal structure of human MTP has provided essential details to understand its poorly characterised mechanisms of action 18 . MTP is a heterodimer consisting of a large α-subunit and a disulfide isomerase β-subunit that acts as a chaperone to stabilise nascent apoB molecules.…”

“…MTP is a heterodimer consisting of a large α-subunit and a disulfide isomerase β-subunit that acts as a chaperone to stabilise nascent apoB molecules. The lipid-binding and transfer functions of MTP are played by its C-terminal domain which harbours a narrow lipid-binding cavity 18 . Upon assembly in the ER, the complexes of self-lipids with CD1 molecules follow the secretory route to the plasma membrane and are then internalised into early or sorting endosomes.…”