RESUMEN
The catabolism of cell-surface heparan sulphate proteoglycans is initiated by endosomal heparanases, which are endoglycosidases that cleave the glycosaminoglycans off core proteins and degrade them to shorter oligosaccharides. We have purified previously four intracellular heparanase activities from Chinese hamster ovary (CHO) cells [Bame, Hassall, Sanderson, Venkatesan and Sun (1998) Biochem. J. 336, 191-200], and in the present study we characterize further the most abundant activity (C1A heparanase). This enzyme purifies as a family of 37-48 kDa proteins from both CHO cells and the rat liver, with the major species being 37 and 40 kDa. Amino acid sequence analysis shows the purified C1A heparanase protein is highly homologous with the N-terminal domain, or FERM domain, of the approximately 80 kDa proteins ezrin, radixin and moesin (ERM proteins, after ezrin-radixin-moesin). This domain, which is also found in erythrocyte protein 4.1, links cytoplasmic proteins to membranes. Antibodies against the FERM domain recognize all the C1A heparanase proteins on Western blots, suggesting that the smaller species are derived from a larger protein. Activity binds to, and is affected by, molecules known to interact with FERM domains, supporting the hypothesis that the intracellular C1A heparanase is the purified FERM domain protein. Since bacterially expressed FERM domains of radixin and moesin lack heparanase activity, and some tryptic peptides generated from the enzyme do not have a match in any ERM protein, it appears that, rather than being derived from ezrin, radixin or moesin, C1A heparanase may be a new member of the FERM domain family.