Recombinant Mouse CD320/TCblR/8D6A Fc Chimera Protein, CF

CD320, also known as the Transcobalamin II receptor (TCblR) and 8D6A, is an approximately 60 kDa transmembrane member of the LDL receptor family (1). Mature mouse CD320 consists of a 180 amino acid (aa) extracellular domain with two cysteine-rich LDLR-A repeats, a 21 aa transmembrane segment, and a 31 aa cytoplasmic domain (2, 3). Within the extracellular domain, mouse CD320 shares 52% aa sequence identity with human and rat CD320. Alternate splicing of mouse CD320 generates an additional isoform with a substitution of the N-terminal 40 aa. CD320 is a heavily glycosylated protein that is expressed as a noncovalent dimer in the kidney, placenta, liver, intestine, and secondary lymphoid organs (3 - 5). It functions as an endocytic receptor for circulating complexes of Cobalamin (Vitamin B₁₂) and Transcobalamin II (3). In the immune system, CD320 is expressed by germinal center B cells and follicular dendritic cells (FDC) and provides a costimulatory signal to B cells during the germinal center reaction (2, 6 - 8). CD320 also cooperates with CD44 in promoting the development of B cell lymphomas (9).