Phosphorylation of optineurin by TBK1 drives fragmentation of Golgi apparatus

Optineurin (OPTN) is a multifunctional adaptor protein involved in various cellular processes. One critical function is maintaining the Golgi complex, as OPTN depletion or its disease-associated mutations leads to Golgi fragmentation. On the other hand, OPTN is known to be phosphorylated by TBK1 to regulate specific cellular processes; however, its role in Golgi regulation remains unclear. Here, we show that expression of a phosphomimetic OPTN mutant, S177E, but not its phospho-deficient mutant, S177A, in HeLa cells effectively results in fragmentation of the Golgi apparatus. Although the effect of S177E appears similar to that of disease-associated OPTN mutations such as E50K, experiments using OPTN double mutants suggest that S177E induces Golgi fragmentation possibly through an E50K-independent pathway. Furthermore, we found that Ser177 phosphorylation in OPTN is enhanced by TBK1, and co-expression of TBK1 with wild-type OPTN induces Golgi fragmentation. Notably, Ser177 phosphorylation leads to the formation of cytoplasmic OPTN puncta, correlating with Golgi fragmentation. Taken together, our data suggest that OPTN phosphorylation by TBK1 induces ectopic OPTN accumulation, leading to Golgi disassembly, possibly via a novel pathway distinct from those associated with disease-related OPTN mutations.