Characterization of Multiple Forms of Small Collagenous Apatite-Binding Proteins in Bone

A number of small (Mrs 25–28 kDa) collagenous apatite-binding (SCAB) proteins that stain blue with “Stains-All” have been isolated from fetal porcine bone by sequential extractions with 4M GuHCl (G1), followed by 0.5M EDTA (E), and again with 4M GuHCl (G2). Following purification under dissociative conditions, two types of SCAB proteins both with approximately one-third of their structure being collagenous, were identified in the EDTA extract. One type, which appears to be a novel protein, was revealed in two forms (SCABs 1 and 2, Mrs 25 and 28 kDa) that were recognized by a monoclonal antibody (MBP-322). The second type, SCAB 3, was also present in two forms; one form (SCAB 3a) having a lower affinity for hydroxyapatite than the other (SCAB 3b). These proteins were resistant to CNBr and displayed the chemical and immunochemical properties of the α1 pN-propeptide of type I collagen. A third form of the propeptide (G2-28K) was a prominent component of the second 4M GuHCl extract. The chromatographic proper...