Interaction of C-Terminal Truncated Human αA-Crystallins with Target Proteins

Background: Significant portion of αA-crystallin in human lenses exists as C-terminal residues cleaved at residues 172, 168, and 162. Chaperone activity, determined with alcohol dehydrogenase (ADH) and βL-crystallin as target proteins, was increased in αA1–172 and decreased in αA1–168 and αA1–162. The purpose of this study was to show whether the absence of the C-terminal residues influences protein-protein interactions with target proteins. Methodology/Principal Findings: Our hypothesis is that the chaperone-target protein binding kinetics, otherwise termed subunit exchange rates, are expected to reflect the changes in chaperone activity. To study this, we have relied on fluorescence resonance energy transfer (FRET) utilizing amine specific and cysteine specific fluorescent probes. The subunit exchange rate (k) for ADH and αA1–172 was nearly the same as that of ADH and αA-wt, αA1–168 had lower and αA1–162 had the lowest k values. When βL-crystallin was used as the target protein, αA1–172 had slightly higher k value than αA-wt and αA1–168 and αA1–162 had lower k values. As expected from earlier studies, the chaperone activity of αA1–172 was slightly better than that of αA-wt, the chaperone activity of αA1–168 was similar to that of αA-wt and αA1–162 had substantially decreased chaperone activity. Conclusions/Significance: Cleavage of eleven C-terminal residues including Arg-163 and the C-terminal flexible arm significantly affects the interaction with target proteins. The predominantly hydrophilic flexible arm appears to be needed to keep the chaperone-target protein complex soluble.