Truncated human betaB1-crystallin shows altered structural properties and interaction with human betaA3-crystallin

The purpose of the study was to determine the effects of truncation of various regions of betaB1-crystallin on its structural properties and stability of heterooligomers formed by wild-type (WT) betaB1 or its deletion mutants with WT betaA3-crystallin. For these analyses, seven deletion mutants of betaB1-crystallin were generated with the following sequential deletions of either N-terminal arm [betaB1(59-252)], N-terminal arm + motif I [betaB1(99-252)], N-terminal arm + motif I + motif II [betaB1(144-252)], N-terminal arm + motif I + motif II + connecting peptide [betaB1(149-252)], C-terminal extension [betaB1(1-234)], C-terminal extension plus motif IV [betaB1(1-190)], or C-terminal extension + motif III + motif IV [betaB1(1-148)]. The betaB1-crystallin became water insoluble on the deletion of C-terminal extension and subsequent deletions of the C-terminal domain (C-terminal extension plus motifs III and IV) while it remained partially soluble on the deletion of the N-terminal domain (N-terminal arm plus motifs I and II). However, circular dichroism spectral analysis showed that the deletion of the N-terminal domain but not the C-terminal domain exhibited relatively greater structural changes in the crystallin. The deletion of the C-terminal domain resulted in a greater exposure and disturbance in the microenvironment of Trp-100, Trp-123, and Trp-126 (localized in the motif II), suggesting a relatively greater role of the C-terminal domain than the N-terminal domain in the structural stability of the crystallin. The deletion of the N-terminal extension in betaB1 resulted in maximum exposure of hydrophobic patches and compact structure and in a maximum loss of subunit exchange with WT betaA3-crystallin compared to deletion of either the C-terminal extension, the N-terminal domain, or the C-terminal domain. The thermal stability results of the heterooligomer of betaB1- plus betaA3-crystallins suggested that oligomers lose their stability on deletion of the C-terminal domain. Together, the results suggested that the N-terminal arm of betaB1-crystallin plays a major role in interaction with betaA3-crystallin during heterooligomer formation, and the solubility of betaB1-crystallin per se and that of the heterooligomer with betaA3-crystallin are dependent on the intact C-terminal domain of betaB1-crystallin.