Does sod1 encode a molecular clock? Mutations that mimic asparagine deamidation inhibit heterodimerization with...

The self-exchange of subunits by protein homodimers is a common protein-protein interaction in vivo. In heterozygous genetic disorders involving homodimeric gene products, both mutant and WT proteins can exchange subunits (heterodimerize). This form of heterodimerization can be analytically challenging to study. In this paper, we used capillary electrophoresis to investigate how deamidation of multiple asparagine residues (to aspartate) in homodimeric Cu, Zn superoxide dismutase-1 (SOD1) affected the rate and free energy of heterodimerization between WT and mutant SOD1 that cause amyotrophic lateral sclerosis (ALS). To model asparagine deamidation, Asn to Asp substitutions were introduced at five Asn residues predicted to undergo the most rapid deamidation in SOD1 (N26D, N131D, N139D, N65D, N19D). This model of penta-deamidated SOD1 did not heterodimerize with WT SOD1 or E100K SOD1 (linked to ALS). In contrast, the quad-variant N26D/N131D/N139D/N19D SOD1 did heterodimerize. These results suggest that the WT SOD1 protein has an intrinsic "timer" or "molecular clock" (as spontaneous Asn deamidation has been described) that effectively stops its heterodimerization after the SOD1 protein has existed in solution for ∼3 months.