Phosphorylation of the α subunit inhibits proteasome assembly and regulates cell division in archaea

The archaea of the order Sulfolobales display a eukaryotic-like cell cycle. Here, we demonstrate that phosphorylation of the α subunit by a eukaryotic-like protein kinase ePK2 affects 26S proteasome assembly and controls cell division in Saccharolobus islandicus . ePK2 exhibits cell cycle-dependent expression at both transcriptional and translational levels. Deletion or overexpression of ePK 2 results in impaired cytokinesis, with the deletion cells being unable to generate single chromosome cells after synchronization and the overexpression cells exhibiting growth retardation and cell enlargement. Interestingly, overexpression of ePK2 leads to a coherent reduction in cellular proteasome activity and degradation of cell division proteins. We identify S200 and T213 of the proteasome α subunit as specific target sites for ePK2 phosphorylation. Functional and structural analyses of site-directed mutants at S200 and T213 suggest that S200 phosphorylation disrupts the assembly of 20S into 26S proteasome whereas T213 phosphorylation interferes the de novo α ring assembly. Collectively, our study uncovers an ingenious and efficient mechanism of proteasome phosphorylation-mediated cell division regulation, a prototype of the eukaryotic cell cycle regulation system, in Sulfolobales archaea.