Regulatory Role of Serine59 in the Oligomeric Dynamics and  Chaperone Function of αB-Crystallin

We previously demonstrated that deletion of the 54FLRAPSW61 sequence, containing the key phosphorylation site Serine 59 (S59), resulted in a two-fold reduction in oligomeric mass and a ten-fold enhancement of αB-crystallin’s chaperone activity. This study examined whether targeted deletion (ΔS59) or phosphomimetic substitution (S59D) of S59 could replicate these effects. Using MALS analysis, we found that the average oligomeric mass decreased from 579 kDa in the wild type (αB-WT) to 556 kDa in ΔS59 and 434 kDa in S59D. Interestingly, the S59A variant had an increased mass of 611 kDa. All variants retained their chaperone function, but their efficiencies varied significantly. Specifically, S59D formed smaller, more polydisperse complexes that effectively suppressed aggregation when interacting with rapidly aggregating substrates. In contrast, ΔS59 and S59A created stable complexes with lysozyme, reducing precipitation and aggregate size. Zeta potential measurements indicated distinct surface charge profiles among the variants, but there was no clear correlation between these charges and their chaperone efficiency. Additionally, cytotoxicity assays conducted on ARPE-19 cells under oxidative stress showed that all S59 variants exhibited comparable protective effects against cell death relative to αB-WT. These results indicate that while S59 is not essential for oligomer formation or chaperone function, it plays an important role in modulating oligomer size and interactions with different substrates. Notably, the effects of S59D were measurable but did not replicate the enhanced functionality observed with the complete deletion of the 54–61 motif, reinforcing the significance of the N-terminal region.