Hepatoma-derived growth factor (HDGF)-related proteins (HRPs) comprise a family of polypeptides named after HDGF, which was identified by its mitogenic activity towards fibroblasts. In the present study, we describe a hitherto unknown HRP, termed HRP-4. The cDNA of bovine HRP-4 (bHRP-4) predicts a polypeptide of 235 amino acids. Northern- and Western-blot analyses of various bovine tissues demonstrated that HRP-4 is only expressed in the testis. Recombinantly produced bHRP-4 and murine HDGF (mHDGF) histidine-tagged polypeptides display growth-factor activity for cultured primary human fibroblasts at an optimum concentration of 1ng/ml in serum-free medium. The growth-factor activity declines with increasing concentrations to reach background levels at 1μg/ml. The expression of the fusion proteins, bHRP-4–green fluorescent protein and mHDGF–green fluorescent protein, in HEK-293 cells demonstrates nuclear localization of the proteins. bHRP-4 and mHDGF bind to the glycosaminoglycans heparin and heparan sulphate, but not to chondroitin sulphate. Affinity constants determined for these interactions are between 6 and 42nM. Comparison of the bHRP-4 amino acid sequence with HRP-1–3 and p52/75/lens epithelium-derived growth factor (LEDGF) shows that these proteins share a conserved N-terminal part of 91 amino acids but have C-termini of different lengths and charge. This demonstrates the modular structure of these proteins and allows its classification into three groups based on charge, size and sequence comparison. HRP-4, HRP-1 and HDGF are small acidic proteins, HRP-3 is a small basic protein, and HRP-2 and p52/75/LEDGF are larger basic proteins.
Abbreviations used: FCS, foetal calf serum; FGF, fibroblast growth factor; GAG, glycosaminoglycan; GFP, green fluorescent protein; HDGF, hepatoma-derived growth factor; hath, homologous with the amino terminus of HDGF; HMG, high-mobility group; HRP, HDGF-related protein; LEDGF, lens epithelium-derived growth factor; ORF, open reading frame; SPR, surface plasmon resonance.
Present address: Institut für Physiologische Biochemie, Universität Bremen, Leobener Strasse NW2/B2, 28359 Bremen, Germany.