Post-translational modifications regulate assembly of early spindle orientation complex in yeast

Mitotic spindle orientation is a critical process in cell division, ensuring proper chromosome segregation. In budding yeast, spindle orientation and chromosome segregation are distinct events. The actin-bound myosin Myo2p plays a role in early spindle orientation, interacting with the adapter Kar9p. Kar9p also binds to Bim1p, a microtubule-associated protein, linking the actin and microtubule cytoskeletons. Prior research has shown that Kar9p and Bim1p form a complex to facilitate spindle orientation. However, the mechanisms regulating this interaction remain unclear. This gap motivated researchers to explore the structural and regulatory features of the Kar9p-Bim1p interaction. No prior work had resolved how post-translational modifications influence this complex. The study aims to clarify how these modifications control complex assembly and disassembly. Understanding these regulatory mechanisms could provide insights into mitotic fidelity. The findings may also inform broader studies on cytoskeletal coordination.

The study aimed to investigate how post-translational modifications regulate the interaction between Kar9p and Bim1p. Researchers focused on the structural and functional aspects of their interaction. The goal was to identify novel regulatory domains and mechanisms. They sought to determine how these modifications influence complex formation. The study also aimed to propose a regulatory framework for complex assembly and disassembly. By solving the crystal structure of the Kar9p-binding motif of Bim1p, the team explored structural determinants. They also examined how sumoylation and phosphorylation affect the interaction. The findings could clarify how these modifications control spindle orientation in yeast.

The researchers used crystallography to determine the structure of the Kar9p-binding motif of Bim1p. They identified a second, novel interaction domain between Kar9p and Bim1p. The team analyzed the effects of sumoylation on Kar9p and phosphorylation on Bim1p. They tested how these modifications influence complex formation and stability. The study included biochemical assays to measure interaction efficiency. Researchers used site-directed mutagenesis to assess functional roles. They also performed phosphorylation experiments using Aurora B/Ipl1p. The results were compared to baseline interactions without modifications.

The crystal structure revealed a novel Kar9p interaction domain on Bim1p. Sumoylation of Kar9p enhanced its interaction with Bim1p. Phosphorylation of Bim1p by Aurora B/Ipl1p reduced complex formation. These modifications regulate the assembly and disassembly of the complex. The study found that sumoylation promotes complex stability. Phosphorylation acts as a regulatory switch to down-regulate interactions. The findings suggest a dynamic regulatory framework for complex formation. These results provide a structural basis for post-translational control.

The study proposes a regulatory framework for the Kar9p-Bim1p complex in yeast. Sumoylation and phosphorylation act as opposing regulators of complex formation. The findings suggest that these modifications control spindle orientation. The structural data support the functional role of the novel interaction domain. The results align with the authors' hypothesis about post-translational regulation. The study does not claim essentiality of any modification. The conclusions are limited to the observed effects in yeast. The authors suggest these findings may inform broader studies on mitotic regulation.