JUVENILE FORM OF MUCOPOLYSACCHARIDOSIS-VI (MAROTEAUX-LAMY SYNDROME) -A C-TERMINAL EXTENSION CAUSES INSTABILITY BUT INCREASES CATALYTIC EFFICIENCY OF ARYLSULFATASE-B

A deficiency of the enzyme arylsulfatase B results in the lysosomal st orage disorder Maroteaux-Lamy syndrome or mucopolysaccharidosis type V I. Severe, intermediate and mild forms of this autosomal recessively i nherited disease can be clinically differentiated. To determine the mo lecular defect in a patient with the intermediate form of the disorder , DNA fragments generated from the patient's mRNA by reverse transcrip tion and subsequent amplification by the polymerase chain reaction wer e subcloned and sequenced. The mRNA transcribed from one allele contai ns a 244-base pair deletion causing a frameshift and a truncation of t he open reading frame. The C-terminal third of the encoded mutant poly peptide has a nonsense sequence. This mutation is due to a deletion of exon 5 in this allele. A silent A to G transition at nucleotide 1191 was present in the same allele, and the second allele was characterize d by a T to C transition at nucleotide 1600 causing a mutation of the translational stop codon to a glutamine codon (534Q) and extending th e encoded polypeptide by 50 amino acids. Stable expression of the 534 Q allele in LTK- cells resulted in a mutant precursor 4 kDa larger tha n the wild-type precursor. The majority of the mutant precursor appear s to be degraded before reaching the trans Golgi. This is consistent w ith an altered polypeptide structure, where a number of missing or mas ked epitopes were observed in an enzyme immunobinding assay using a pa nel of monoclonal antibodies. Immunoquantification analysis showed tha t epitopes were most likely masked, as missing epitopes could be refor med by binding the mutant protein to a polyclonal antibody of arylsulf atase B. It is suggested that the additional amino acids at the C term inus of the arylsulfatase B polypeptide induce a protein conformationa l change. 534Q mutant polypeptide escaping degradation is sorted to d ense lysosomes. The mutant polypeptide has an approximately 9-fold hig her catalytic efficiency than wild-type arylsulfatase B.