Recombinant Human URB Protein, CF

URB (upregulated in BRS-3 deficient mice) is a 150 kDa, secreted glycoprotein that belongs to the sushi-repeat-containing protein superfamily (1, 2). Sushi repeats, otherwise known as short consensus repeats (SCRs), are 60 amino acid (aa) sequences usually involved in protein-protein interaction. They are characterized by the presence of four Cys, two Pro, one Gly and one Trp (3). Human URB is synthesized as a 950 aa precursor that contains a 21 aa signal sequence and a 929 aa mature region. The mature molecule contains three extended sushi/SCR domains of approximately 140 aa each. They bear resemblance to the fifth sushi-repeat in human SPRX (4). The three lie between aa 141 - 281, 615 - 760, and 771 - 913, respectively. Between the first and second SCR lie two amino acid-rich regions, a Thr-rich domain (aa 347 - 404), and a Lys-rich domain (aa 487 - 588). Three potential N-linked glycosylation sites exist in the last two SCR’s, while six potential bipartite nuclear localization signals (NLS) occur between aa 420 - 780. There are two potential alternate splice forms for human URB. One is 594 aa in length, and shows a simple truncation at Ser594. This effectively removes the second and third SCRs and two bipartite NLS (5). The second is 553 aa in length and shows a simple truncation after Lys553. This eliminates four bipartite NLSs, the second and third SCRs, and part of the Lys-rich domain (6). Full-length human URB is 83%, 84% and 87% aa identical to rat, mouse and bovine URB, respectively. URB is found in chondrocytes and appears to be downregulated upon CFU-Fibroblast differentiation (1). Thus, it may play a role in skeletogenesis.