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A DNA sequence encoding the human IGFBP-3 protein sequence (containing the signal peptide sequence, and the mature human IGFBP-3 sequence) was expressed in modified human 293 cells.

IGFBP-3 is a member of the insulin-like growth factor binding protein (IGFBP) family. This family includes IGFBP-1 to IGFBP-6 that bind to the insulin-like growth factors (IGFs), thus regulating the half-life, activity, transport, and tissue distribution of the IGFs. Structurally, the IGFBP family of proteins is defined by highly homologous amino and carboxy terminal domains with a structurally diverse central region. The IGF binding motifs exist in the amino and carboxy terminal regions. The carboxy terminal domains may also be involved in interactions with numerous other molecules that may modulate IGF-binding as well as conferring IGF-independent function. IGFBP-3 is the most abundant IGFBP found in human serum where it is predominantly found as a ternary complex with IGF-I or IGF-II and the acid-labile subunit (ALS). This complex accounts for the majority of circulating IGF. Functionally, IGFBP-3 has been observed to exhibit both cell growth promoting and inhibiting effects depending on its effect on the bioavailability of IGF, and its IGF-independent function. IGFBP-3 is a 264 amino acid glycoprotein. In addition to glycosylation IGFBP-3 can also be phosphorylated and these post-translational modifications influence functional activity of IGFBP-3 by modulating tissue targeting, cell interaction and susceptibility to proteolytic cleavage. IGFBP-3 contains three potential N-linked glycosylation sites (Asn-X-Ser/Thr) located in the central linker domain at Asn 89 Asn 109, and Asn 172, while phosphorylation occurs at Ser 111 and 113. For a recent review of the IGFBPs please refer to Bach LA (2005) Trends Endocrinol Metab. 16(5):228-234 or Firth SM (2002) Endocr Rev 23(6):824-854.