ARGININOSUCCINATE LYASE & D II CRYSTALLIN: INTRAGENIC COMPLEMENTATION AND CATALYSIS. P. Lynne Howell1,2, Mary A. Turner1, Mona Abu Abed1, Rod R. McInnes1,3, 1Hospital for Sick Children, Toronto, Ontario, M5G 1X8, 2Departments of Biochemistry and 3Molecular and Medical Genetics, University of Toronto, Toronto, Ontario, M5S 1A8
Argininosuccinate lyase (ASL) and ( II crystallin are homologous proteins with different biological functions but identical enzymatic activities. Both proteins catalyze the reversible cleavage of argininosuccinate into arginine and fumarate. Genetic defects in ASL result in argininosuccinic aciduria, the second most common urea cycle disorder. In addition to the 13 unique ASL mutations that have been identified, intragenic complementation has also been demonstrated at the ASL locus. This is a phenomenon that occurs when a multimeric protein is formed from monomers produced by two different mutant alleles of the same gene. Of the complementing alleles found to date, a combination of the Q286R allele with the D87G allele is the most successful with the greatest restoration of activity. While the homotetrameric mutant proteins were inactive, the hybrid multimers exhibit approximately 30% of wild-type activity.
In order to understand how the enzymatic activity is restored and to elucidate the catalytic mechanism for the reaction, we have determined the X-ray crystal structures of human ASL and wild-type and several duck d II crystallin mutants. The structural results suggest that the restoration of enzymatic activity in the hybrid tetramers is due to the reconstruction of one or more "native" like active sites and that Asp 87 and His 89 are involved in binding the arginine end of the substrate molecule.
(Supported by grants from Medical Research Council of Canada and The Natural Science and Engineering Research Council of Canada)