Nuclear import of αB-crystallin is phosphorylation-dependent and hampered by hyperphosphorylation of the myopathy-related mutant R120G

Phosphorylation modulates the functioning of alpha B-crystallin as a molecular chaperone. We here explore the role of phosphorylation in the nuclear import and cellular localization of alpha B-crystallin in HeLa cells. Inhibition of nuclear export demonstrated that phosphorylation of alpha B-crystallin is required for import into the nucleus. As revealed by mutant analysis, phosphorylation at Ser-59 is crucial for nuclear import, and phosphorylation at Ser-45 is required for speckle localization. Co-immunoprecipitation experiments suggested that the import of alpha B-crystallin is possibly regulated by its phosphorylation-dependent interaction with the survival motor neuron (SMN) protein, an important factor in small nuclear ribonucleoprotein nuclear import and assembly. This interaction was supported by co-localization of endogenous phosphorylated alpha B-crystallin with SMN in nuclear structures. The cardiomyopathy-causing alpha B-crystallin mutant R120G was found to be excessively phosphorylated, which disturbed SMN interaction and nuclear import, and resulted in the formation of cytoplasmic inclusions. Like for other protein aggregation disorders, hyperphosphorylation appears as an important aspect of the pathogenicity of alpha B-crystallin R120G.