The heterogeneity of link proteins isolated from human articular cartilage proteoglycan aggregates.

Link proteins, isolated from proteoglycan aggregates prepared from human articular cartilage, were studied by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and isoelectric focusing. When subjected to the former technique the native link proteins resolve into three components of molecular weights 48,000, 44,000, and 41,000. Link protein, isolated following clostripain digestion of proteoglycan aggregate, is present as a single component of molecular weight 41,000. Under all conditions of isoelectric focusing tested, the native link proteins resolve into at least nine subcomponents having pI values between 6.0 and 7.0. The clostripain-treated link protein resolves into at least five subcomponents which have pI values similar to those of the more basic subcomponents observed in the native molecules. One source of heterogeneity contributing to both isoelectric focusing profiles is variation in sialic acid content, since neuraminidase treatment of the link protein preparations produces a shift to subcomponents with more basic pI. The electrophoretic data are consistent with the two larger link protein components representing the same protein core, but being substituted to different degrees with oligosaccharide chains that may have variable sialic acid contents. The smallest link protein component, which may be derived from either of the larger moieties by limited proteolytic cleavage, is also substituted with sialic acid-containing oligosaccharides.