Disruption of a Salt Bridge Dramatically Accelerates Subunit Exchange in Duck δ2 Crystallin

Abstract

Intragenic complementation is a unique property of oligomeric enzymes with which to study subunit-subunit interactions. Complementation occurs when different subunits, each possessing distinct mutations that render the individual homomutant proteins inactive, interact to form a heteromutant protein with partial recovery of activity. In this paper, complementation events between human argininosuccinate lyase (ASL) and its homolog, duck ␦2 crystallin, were characterized. Different active site mutants in ␦2 crystallin complement by the regeneration of native-like active sites as reported previously for ASL. The complementarity of the ASL and ␦2 crystallin subunit interfaces was illustrated by the in vivo formation of active hybrid tetramers from inactive ASL and inactive ␦2 crystallin mutants. Subunits of both ASL and ␦2 crystallin do not dissociate and reassociate in vitro at room temperature, even after 6 days of incubation, indicating that the multimerization interface is very strong. However, disruption of a salt bridge network in the tetrameric interface of ␦2 crystallin caused a drastic acceleration of subunit dissociation. Double mutants combining these interface mutants with active site mutants of ␦2 crystallin were able to dissociate and reassociate to form active tetramers in vitro within hours. These results suggest that exchange of subunits may occur without unfolding of the monomer. Intragenic complementation in these interface mutants occurs by reintroducing the native salt bridge interaction upon hetero-oligomerization. Our studies demonstrate the value of intragenic complementation as a tool for investigating subunit-subunit interactions in oligomeric proteins.The majority of naturally occurring proteins exist as oligomers. While surveying the Escherichia coli genome, Goodsell and Olsen (1) found that about 80% of the proteins were at least dimers if not higher order multimers. The primary amino acid sequence of a protein not only has to contain the information required to fold the polypeptide chain into its three-dimensional structure properly but also the subunit associations that allow it to homo-or hetero-oligomerize to its final quaternary state. Although there have been many advances in understanding how individual protein units fold (2-4), the study of how oligomeric proteins coordinate folding with the assembly of subunits is still in its infancy. A unique property of oligomeric enzymes is their potential ability to exhibit intragenic complementation. Intragenic complementation is a phenomenon that can occur between different mutant subunits within a multimeric enzyme. Complementation occurs when certain combinations of mutant alleles produce an enzyme with greater catalytic activity than is observed in the homozygous state of either mutant. The occurrence of intragenic complementation in well characterized multimeric enzymes provides a valuable tool to study the subunit-subunit interactions of proteins.Argininosuccinate lyase (ASL, 1 EC 4.3.2.1) is a ubiquitous enzyme that catalyzes the rev...

Interacting Residues in an Activated State of a G Protein-coupled Receptor

“…It has been reported that a predicted salt-bridge interaction between Arg 173 and Glu 318 in a GPCR was not interchangeable as judged in studies on the receptor containing the reciprocal mutation (43). In addition, although the interaction between Arg 302 and Glu 330 was identified in the crystal structure of crystallin, the R302E/E330R reciprocal mutation showed 31% of WT activity, which is equivalent to the activity of E330R mutant (44). These results suggest that a saltbridge interaction is not always interchangeable and may account for the suppression of constitutive activity by the introduction of N205D/ Y266K but not N025K/Y266D into the P258L/S259L constitutively active mutant.…”

Functional Complementation in Yeast Allows Molecular Characterization of Missense Argininosuccinate Lyase Mutations

“…9, A and B). The second occurs when a stable mutant subunit and an unstable mutant subunit form a hybrid protein with enough overall stability for catalysis to occur (24).…”

Salt Modulates Oligomerization Properties of hRad51 and hRad52 Proteins

“…Disruption of salt bridges thereby facilitates subunit movement [65] and thereby modulates stability of higher order oligomeric protein structures. Salt at high concentration is known to inhibit self assembly of individual subunits in protein oligomers [43,66].…”