Abstract
<h3>Abstract</h3> Cellular uptake of vitamin B<sub>12</sub> in humans is mediated by the endocytosis of the B<sub>12</sub> carrier protein transcobalamin (TC) via its cognate cell surface receptor TCblR (or CD320), encoded by the <i>CD320</i> gene(1). Because <i>CD320</i> expression is associated with the cell cycle and upregulated in highly proliferating cells such as cancer cells(2–4), this uptake route is a potential target for cancer therapy(5). We developed and characterized four camelid nanobodies that bind TC or the interface of the TC:TCblR complex with nanomolar affinities. We determined X-ray crystal structures of all four nanobodies in complex with TC:TCblR, which enabled us to map their binding sites. When conjugated to a toxin, three of these nanobodies are capable of inhibiting the growth 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 (TCNB4) using fluorescent light microscopy. The co-crystal structures of TC:TCblR with another nanobody (TCNB34) revealed novel features of the interface of TC and the LDLR-A1 domain of TCblR. Our findings rationalize the structural basis for a decrease in affinity of TC-B<sub>12</sub> binding caused by the TCblR-Glu88 deletion mutant.
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