ERK1c is an alternatively spliced isoform of ERK1 that specifically regulates mitotic Golgi fragmentation, which allows division of the Golgi during mitosis. We have previously shown that ERK1c translocates to the Golgi during mitosis where it is activated by a resident MEK1b to induce Golgi fragmentation. However, the mechanism of ERK1c functions in the Golgi remained obscure. Here, we searched for ERK1c substrates and identified HOOK3 as a mediator of ERK1c-induced mitotic Golgi fragmentation, which requires a second phosphorylation by AuroraA for its function. In cycling cells, HOOK3 interacts with microtubules (MTs) and links them to the Golgi. Early in mitosis, HOOK3 is phosphorylated by ERK1c and later by AuroraA, resulting in HOOK3 detachment from the MTs, and elevated interaction with GM130. This detachment modulates Golgi stability and allows fragmentation of the Golgi. This study demonstrates a novel mechanism of Golgi apparatus destabilization early in mitosis to allow mitotic progression.
© 2021 The Author(s).

α-Tubulin acetyltransferase 1 (αTAT1) controls reversible acetylation on Lys40 of α-tubulin and modulates multiple cellular functions. αTAT1 depletion induced morphological defects of touch receptor neurons in Caenorhabditis elegans and impaired cell adhesion and contact inhibition in mouse embryonic fibroblasts, however, no morphological or proliferation defects in human RPE-hTERT cells were found after αTAT1-specific siRNA treatment. Here, we compared the effect of three αTAT1-specific shRNAs on proliferation and morphology in two human cell lines, HeLa and A549. The more efficient two shRNAs induced mitotic catastrophe in both cell lines and the most efficient one also decreased F-actin and focal adhesions. Further analysis revealed that αTAT1 downregulation increased γ-H2AX, but not other DNA damage markers p-CHK1 and p-CHK2, along with marginal change in microtubule outgrowth speed and inter-kinetochore distance. Overexpression of αTAT1 could not precisely mimic the distribution and concentration of endogenous acetylated α-tubulin (Ac-Tu), although no overt phenotype change was observed, meanwhile, this could not completely prevent αTAT1 downregulation-induced deficiencies. We therefore conclude that efficient αTAT1 downregulation could impair actin architecture and induce mitotic catastrophe in HeLa and A549 cells through mechanisms partly independent of Ac-Tu.