Decreasing the homodimer interaction: a common mechanism shared by the deltaG91 mutation and deamidation in betaA3-crystallin

Abstract

Purpose: Cataracts can be broadly divided into two types: congenital cataracts and age-related cataracts. DeltaG91 is a previously discovered congenital mutation in betaA3-crystallin that impairs protein solubility. On the other hand, the deamidation of beta-crystallin is a significant feature in aged and cataractous lenses. Several deamidation sites were also identified in betaA3-crystallin. The present study is to compare the functional consequence of DeltaG91 mutation and the deamidation of betaA3-crystallin in terms of folding properties and protein-protein interaction.

Methods: Protein secondary structure and hydrophobic properties were investigated by in silica analysis of the wild type and mutants sequences. Full-length betaA3-crystallin was cloned into a mammalian two-hybrid system in order to investigate protein-protein interactions. Deletion and deamidation were introduced by site-directed mutagenesis protocols. Both the Q85 and Q180 deamidation sites were substituted with glutamic acid residues to mimic deamidation. Different combinations of plasmid constructs were transfected in HeLa cells, and changes of protein-protein interactions were analyzed by the luciferase assay.

Results: Bioinformatics prediction suggested that DeltaG91 mutation alters both the predicted secondary structure and hydrophobic character of betaA3-crystallin, while deamidation only exhibits minimal effects. Mammalian two-hybrid results indicated that both DeltaG91 mutation and Q85/Q180 deamidation could significantly decrease the interaction of the betaA3-crystallin homodimer.

Conclusion: Our results provided evidence that both mutations involved in congenital cataracts and deamidation in aged lenses commonly altered protein-protein interaction between human lens betaA3-crystallins, which may lead to protein insolubilization and contribute to cataracts.

Figure 1

Predicted secondary structures of the wild type and the mutant βA3-crystallin. The following sequences were used for secondary structures prediction: wild type (Wild_type); glutamine that residues at position 85 is replaced by a glutamic acid residue (Q85E); glutamine that residues at position 180 is replaced by a glutamic acid residue (Q180E); βA3-crystallin double deamidated mutant (Q85EQ180E); or glycine residue at position 91 is removed (G91Del). The secondary structure types are designated as follows (according to DSSP program): H=alpha helix; B=residue in isolated beta-bridge; E=extended strand, participates in beta ladder; G=3-helix (3/10 helix); I=5 helix (pi helix); T=hydrogen bonded turn; S=bend; C=the rest.

Figure 2

Protein hydrophobicity analysis of the wild type and the mutant βA3-crystallin. Kyte-Doolittle hydrophobicity plot of wild type βA3-crystallin (A), ΔG91 mutant (B), and Q85E/Q180E mutant (C). X-axis represents amino acid position of, and y-axis represents hydropathy value in a window size of 5. The region of interest is marked by blue box (for ΔG91 mutant) or white boxes (Q85E/Q180E mutant).

Figure 3

Luciferase activities for detection of protein–protein interactions involving various ΔG91 mutants. Luciferase activity values are expressed as fold activation relative to the basal control (pCMV-AD+pCMV-BD). Various plasmid constructs were co-transfected as labeled. Data represent the mean±SEM of results in three independent experiments. Group differences were all compared with wild type homodimer interaction (pAD-βA3+pBD-βA3). The asterisk indicates a p<0.05 and the double asterisk indicates a p<0.01.

Figure 4

Luciferase activities for detection of protein–protein interactions involving various βA3-crystallin deamidation mutants. Luciferase activity values are expressed as fold activation relative to the basal control (pCMV-AD+pCMV-BD). Various plasmid constructs were co-transfected as labeled. Data represent the mean±SEM of results in three independent experiments. Group differences were all compared with wild type homodimer interaction (pAD-βA3+pBD-βA3). The asterisk indicates a p<0.05 and the double asterisk indicates a p<0.01.