7qbg | pdb_00007qbg
TC:CD320 in complex with nanobody TC-Nb4
Structural highlights
DiseaseTCO2_HUMAN Defects in TCN2 are the cause of transcobalamin II deficiency (TCN2 deficiency) [MIM:275350. This results in various forms of anemia. FunctionTCO2_HUMAN Primary vitamin B12-binding and transport protein. Delivers cobalamin to cells. Publication Abstract from PubMedCellular uptake of vitamin B(12) in humans is mediated by the endocytosis of the B(12) carrier protein transcobalamin (TC) via its cognate cell surface receptor TCblR, encoded by the CD320 gene. Because CD320 expression is associated with the cell cycle and upregulated in highly proliferating cells including cancer cells, this uptake route is a potential target for cancer therapy. We developed and characterized four camelid nanobodies that bind holo-TC (TC in complex with B(12) ) or the interface of the human holo-TC:TCblR complex with nanomolar affinities. We determined X-ray crystal structures of these nanobodies bound to holo-TC:TCblR, which enabled us to map their binding epitopes. When conjugated to the model toxin saporin, three of our nanobodies caused growth inhibition of HEK293T cells and therefore have the potential to inhibit the growth of human cancer cells. We visualized the cellular binding and endocytic uptake of the most potent nanobody (TC-Nb4) using fluorescent light microscopy. The co-crystal structure of holo-TC:TCblR with another nanobody (TC-Nb34) revealed novel features of the interface of TC and the LDLR-A1 domain of TCblR, rationalizing the decrease in the affinity of TC-B(12) binding caused by the Delta88 mutation in CD320. Generation of nanobodies targeting the human, transcobalamin-mediated vitamin B(12) uptake route.,Bloch JS, Sequeira JM, Ramirez AS, Quadros EV, Locher KP FASEB J. 2022 Apr;36(4):e22222. doi: 10.1096/fj.202101376RR. PMID:35218573[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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