Structural characterization of inter-alpha-inhibitor - Evidence for an extended shape

Inter-alpha-inhibitor (I alpha I) is a 180-kDa serum protein consisting of three polypeptides, Two of these, the heavy chains 1 and 2 (H1 and H2), are of 75-80 kDa and have similar amino acid sequences. The third polypeptide, bikunin, has a molecular mass of 25 kDa and contains a 7-kDa chondroitin s ulfate chain that is covalently linked to the C-terminal amino acid residue s of H1 and H2, I alpha I has been shown to be required for the formation o f the hyaluronan-containing extracellular matrix of certain cell types. How I alpha I exerts this function is not known, but it appears that upon inte raction with cells, the heavy chains are released and become covalently lin ked to hyaluronan, Our results indicate that I alpha I and its heavy chains are extended molecules; thus, upon electron microscopy, I alpha I appeared to consist of two globular domains connected by a thin structure 31-nm lon g and the isolated heavy chains of a globular domain and a "tail" about 15- nm long. Analysis of the heavy chains by partial proteolysis showed that th e C-terminal halves are particularly sensitive to hydrolysis indicating tha t they are loosely folded. Furthermore, electron microscopy showed that par tially degraded heavy chains lacked the extended regions. Taken together, t hese results suggest that the N-terminal half of the heavy chains forms a g lobular domain, whereas the other half has an extended and loosely folded s tructure.