Domain organization of Mac-2 binding protein and its oligomerization to linear and ring-like structures

The multidomain Mac-2 binding protein (M2BP) is present in serum and in the extracellular matrix in the form of linear and ring-shaped oligomers, whic h interact with galectin-3, fibronectin, collagens, integrins and other lar ge glycoproteins. Domain 1 of M2BP (M2BP-1) shows homology with the cystein e-rich SRCR domain of scavanger receptor. Domains 2 and 3 are related to th e dimerization domains BTB/POZ and IVR of the Drosophila kelch protein. Rec ombinant M2BP, its N-terminal domain M2BP-1 and a fragment consisting of pu tative domains 2, 3 and 4 (M2BP-2,3,4) were investigated by scanning transm ission electron microscopy, transmission electron microscopy, analytical ul tracentrifugation and binding assays. The ring oligomers formed by the inta ct protein are comprised of approximately 14 nm long segments composed of t wo 92 kDa M2BP monomers. Although the rings vary in size, decamers predomin ate. The various linear oligomers also observed are probably ring precursor s, dimers predominate. M2BP-1 exhibits a native fold, does not oligomerize and is inactive in cell attachment. M2BP-2,3,4 aggregates to heterogeneous, protein filled ring-like structures as shown by metal shadowed preparation s. These aggregates retain the cell-adhesive potential indicating native fo lding. It is hypothesized that the rings provide an interaction pattern for multivalent interactions of M2BP with target molecules or complexes of lig ands. (C) 1999 Academic Press.