Glycosaminoglycans are degraded sequentially by a series of enzymes in lysosomes. Low or missing activity in any of these enzymes is responsible for each of the diseases known as mucopolysaccharidoses (MPS). Deficiency in the 4-sulfatase leads to an accumulation of the degradation products of dermatan sulfate and the clinical condition MPS VI or Marateaux-Lamy syndrome. The enzyme is targeted to the lysosome by a phosphorylated mannose tag. A 41 residue N- terminal sequence is lost on entering the endoplasmic reticulum. This precursor protein is then further clipped at two sites to yield the mature form. The human enzyme has been expressed at high levels in Chinese Hamster Ovary cells with a view to its use in enzyme replacement therapy. We have crystallised the 492 residue precursor enzyme which contains a considerable amount of N-linked carbohydrate. The structure was solved using MIR methods and refined at a resolution of 2.5Å, using diffraction data recorded at -150 deg. C. The active site and possible catalytic residues have been identified. Some of the attached carbohydrate is included in the present model.
Supported by the Australian Research Council and the National Health and Medical Research Council.