Variable zinc coordination in endostatin

Endostatin is a proteolytic fragment of collagen XVIII that potently inhibi ts angiogenesis and tumour growth. Human endostatin contains a zinc ion, bo und near the N terminus, which was not observed in the original structure o f mouse endostatin at pH 5. Controversial data exist on the role of this zi nc ion in the anti-tumour activity. We report two new crystal structures of mouse endostatin at pH 8.5 with bound zinc. One crystal form shows a metal ion coordination similar to that in human endostatin (His132, His134, His1 42, Asp207), but the conformation of the N-terminal segment is different. I n the other crystal form, Asp136 replaces His132 as a zinc ligand. Site-dir ected mutagenesis of zinc-binding residues demonstrates that both coordinat ion geometries occur in solution. The large degree of structural heterogene ity of the zinc-binding site has implications for endostatin function. We c onclude that zinc is likely to play a structural rather than a critical fun ctional role in endostatin. (C) 2000 Academic Press.