GLYCOSAMINOGLYCAN BINDING CHARACTERISTICS OF THE INSULIN-LIKE GROWTH FACTOR-BINDING PROTEINS

Interactions between the IGF-binding proteins (IGFBPs) and glycosamino glycans (GAGs) such as heparin may be involved in the regulatory contr ol of IGF exerted by the IGFBPs at the level of the extracellular matr ix and capillary endothelium, although the precise mechanisms of this remain uncertain. We have searched primary sequences of human, rat and bovine IGFBPs-1 to -6 for putative GAG-binding consensus sequences (X BBXBX and XBBBXXBX, where B represents any basic amino acid and X is u ndefined). At least one such sequence was identified in each IGFBP exa mined except human and rat IGFBP-4 and rat IGFBP-6, with IGFBP-5 conta ining three GAG-binding consensus sequences. Additionally, the bovine IGF type II receptor was found to contain two such sequences in the in tracellular region. Affinity of the IGFBP preparations for heparin was examined experimentally by affinity chromatography using pooled fract ions of fetal and adult ovine plasma obtained by size exclusion chroma tography. Pooled fractions of 150 kDa (containing IGFBP-3 alone by IGF ligand blot analysis) and 40-50 kDa (containing IGFBPs-3 and -2, toge ther with proteins of 29, 24 and 25-28 kDa which may include IGFBP-4 a nd IGFBPs-1, -5 and -6) were found to bind strongly to the matrix nece ssitating high salt concentrations for their elution; however, in cont rast, a > 200 kDa fraction containing the soluble form of the type II receptor failed to bind. Recombinant human non-glycosylated IGFBP-3 al so bound strongly to the affinity adsorbent. No evidence of dissociati on of bound IGF from binding protein complexes by association with the matrix was obtained from this experiment. This study provides a molec ular basis for the interaction of IGFBPs with matrix GAGs, although pr ecise mechanisms by which this may influence IGF bioactivity at the ce llular level remain to be established.