Cloning and sequence determination of bovine insulin‐like growth factor binding protein‐2 (IGFBP‐2): Comparison of its structral and functional properties with IGFBP‐1

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Abstract
Insulin‐like growth factor binding proteins (IGFBPs) are secreted by several cell types and can modify IGF actions. Mandin‐Darby Bovine Kidney (MDBK) cells have been shown to secrete a 34,000 Da form of IGF binding protein whose N‐terminal sequence is similar to a form of IGFBP purified from rat BRL‐3A cells that has recently been named IGFBP‐2. These studies report the complete amino acid sequence of bovine IGFBP‐2 and compare its functional properties with human IGFBP‐1. The protein is 81% identical to rat IGFBP‐2. When compared with both rat IGFBP‐2 and human IGFBP‐1, the positions of all 18 cysteine residues are conserved. Similarly an RGD sequence is present near the carboxyl terminus in both proteins. IGFBP‐2 has a higher affinity for IGF‐II than for IGF‐I and its affinity for both forms of IGF is greater than for human IGFBP‐1. Like IGFBP‐1 the protein can enhance the DNA synthesis response of porcine aortic smooth muscle cells to IGF‐I; however, IGFBP‐2 was much less potent. The maximum potentiation of the IGF‐mediated mitogenic response that could be achieved was approximately 42% that of IGFBP‐1. This potentiation is dependent upon a factor contained in platelet poor plasma and if this factor is omitted from the incubation medium, IGFBP‐2 inhibits DNA synthesis. The purification of IGFBP‐2 will allow more detailed comparisons to be made between it and other forms of IGFBPs in physiologic test systems.

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