Maple syrup urine disease (MSUD) is caused by a deficiency of the mitochondrial branched-chain α-keto acid dehydrogenase (BCKAD) complex. The multienzyme complex comprises five enzyme components, including the E1 decarboxylase with a heterotetrameric (α2β2) structure. Four unrelated Hispanic-Mexican MSUD patients with the intermediate clinical phenotype were diagnosed 7 to 22 mo after birth during evaluation for developmental delay. Three of the four patients were found homozygous for G to A transition at base 895 (exon 7) of the E1α locus, which changes Gly-245 to Arg (G245R) in that subunit. The remaining patient was homozygous for T to G transversion at base 1,253 in the E1α gene, which converts Phe-364 to Cys (F364C) in the gene product. Transfection studies in E1α-deficient lymphoblasts indicate that both G245R and F364C mutant E1α subunits were unable to significantly reconstitute BCKAD activity. Western blotting showed that both mutant E1α subunits in transfected cells failed to efficiently rescue the normal E1β through assembly. The putative assembly defect was confirmed by pulse-chase labeling of E1 subunits in a chaperone-augmented bacterial overexpression system. The kinetics of initial assembly of the G245R E1α subunit with the normal E1β was shown to be slower than the normal E1α. No detectable assembly of the F364C E1α subunit with normal E1β was observed during the 2 h chase. Small amounts of recombinant mutant E1 proteins were produced after 15 h induction with isopropyl thiogalactoside and exhibited very low or no E1 activity. Our study establishes that G245R and F364C mutations in the E1α subunit disrupt both the E1 heterotetrameric assembly and function of the BCKAD complex. Moreover, the results suggest that the G245R mutant E1α allele may be important in the Hispanic-Mexican population.
- Branched-chain α-keto acid dehydrogenase complex
- Defective heterotetrameric assembly
- E1 dysfunction
- Molecular chaperones
- Pulse-chase labeling
ASJC Scopus subject areas