Submitted May 27, 2008
Accepted August 3, 2008
The protein and the gene encoding the receptor for the cellular uptake of transcobalamin bound cobalamin
Edward V. Quadros*, Yasumi Nakayama, and Jeffrey M. Sequeira
Department of Cell Biology, SUNY- Downstate Medical Center, Brooklyn, NY, United States
Department of Medicine, SUNY-Downstate Medical center, Brooklyn, NY, United States
* Corresponding author; email: edward.quadros{at}downstate.edu.
The transcobalamin (TC, TCII) receptor (TCblR) on the plasma membrane binds TC- cobalamin (Cbl) and internalizes the complex by endocytosis. This receptor was purified from human placental membranes by affinity chromatography. Tryptic digest of the protein extracted from a SDS-PAGE gel and subjected to LC-MS-MS analysis identified four peptides that matched with a membrane protein in the data bank. TCblR belongs to the LDL receptor family with two LDL receptor type A domains separated by a complement like cysteine rich region. The 282 amino acid sequence includes a signal peptide of 31 residues, extracellular domain of 198 residues, a transmembrane region of 21 residues and a cytoplasmic domain of 32 residues. The binding of TC-Cbl does not require the cytoplasmic domain or its orientation in the plasma membrane because the recombinant extracellular domain produced in mammalian cells binds TC-Cbl with high affinity and specificity. The protein is heavily glycosylated and accounts for the 58kDa size by SDS-PAGE. The human gene first identified as 8D6A and more recently as CD 320 encoding TCblR is located at p13.2 on the short arm of chromosome 19, spans a length of 6.224 kb and is comprised of 5 exons and 4 introns.
