[5][6] MTTP encodes the large subunit of the heterodimeric microsomal triglyceride transfer protein (MTP).

Protein disulfide isomerase (PDI) completes the heterodimeric MTP, which has been shown to play a central role in lipoprotein assembly.

[citation needed] MTP adds triglycerides to nascent chylomicrons in the intestine, and to VLDL in the liver.

[7] The large subunit of MTP, also known as the alpha subunit, contains an N-terminal half beta barrel, an alpha helix and a C-terminal lipid binding site that lies between two beta pleated sheets.

The heterodimer is instead retained in the endoplasmic reticulum due to the presence of a C-terminal KDEL motif on the PDI beta subunit.