Recombinant Mouse R-Spondin 4 Protein

R-Spondin 4 (RSPO4, roof plate-specific spondin 4), also called cysteine-rich and single thrombospondin domain containing-4 (Cristin 4), is an ~33 kDa secreted heparin-binding protein that shares ~35% amino acid (aa) identity with three other R-Spondin family members (1-3). All are positive modulators of Wnt/ beta-catenin signaling, but R-Spondin 4 may be somewhat weaker than other R-Spondins (2). R‑Spondins regulate Wnt/ beta-catenin by competing with the Wnt antagonist DKK-1 for binding to the Wnt co-receptors LRP-6 and Kremen, reducing their DKK‑1‑mediated internalization (1, 4). Like other R‑Spondins, mouse R-Spondin 4 (234 aa) contains a signal sequence (aa 1-19), two adjacent cysteine-rich furin-like domains (aa 85-128) with one potential tyrosine phosphorylation site (aa 114), followed by a thrombospondin (TSP-1) motif (aa 137‑197) and a region rich in basic residues (aa 199‑234). The furin-like domains are sufficient for beta-catenin stabilization (2). Mature mouse R‑Spondin 4 shares 81%, 97%, 79%, 77% and 76% aa identity with human, rat, bovine, equine and canine R-Spondin 4, respectively. There is one potential isoform where Arg substitutes for the C‑terminal 82 amino acids (5). Each R‑Spondin has a distinct expression pattern (6). In the mouse, R‑Spondin 4 mRNA is found during development of limb bud mesenchyme, nail beds, heart and teeth (6‑8). In humans, mutations of R‑Spondin 4 have been found to cause anonychia, a condition in which fingernails and toenails are absent (8‑10).