Tyrosines in the Kinesin-5 Head Domain Are Necessary for Phosphorylation by Wee1 and for Mitotic Spindle Integrity

Mitotic spindle assembly and maintenance relies on kinesin-5 motors that act as bipolar homotetramers to crosslink microtubules [1], [2], [3], [4] and [5]. Kinesin-5 motors have been the subject of extensive structure-function analysis [5], but the regulation of their activity in the context of mitotic progression remains less well understood [2]. We report here that Drosophila kinesin-5 (KLP61F) is regulated by Drosophila Wee1 (dWee1). Wee1 tyrosine kinases are known to regulate mitotic entry via inhibitory phosphorylation of Cdk1 [6], [7], [8], [9] and [10]. Recently, we showed that dWee1 also plays a role in mitotic spindle positioning through γ-tubulin and spindle fidelity through an unknown mechanism [11]. Here, we investigated whether a KLP61F-dWee1 interaction could explain the latter role of dWee1. We found that dWee1 phosphorylates KLP61F in vitro on three tyrosines within the head domain, the catalytic region that mediates movement along microtubules. In vivo, KLP61F with tyrosine→phenylalanine mutations fails to complement a klp61f mutant and dominantly induces spindle defects similar to ones seen in dwee1 mutants. We propose that phosphorylation of the KLP61F catalytic domain by dWee1 is important for the motor's function. This study identifies a second substrate for a Wee1 kinase and provides evidence for phosphoregulation of a kinesin in the head domain.     

Dying Cells Protect Survivors from Radiation-Induced Cell Death in Drosophila

We report a phenomenon wherein induction of cell death by a variety of means in wing imaginal discs of Drosophila larvae resulted in the activation of an anti-apoptotic microRNA, bantam. Cells in the vicinity of dying cells also become harder to kill by ionizing radiation (IR)-induced apoptosis. Both ban activation and increased protection from IR required receptor tyrosine kinase Tie, which we identified in a genetic screen for modifiers of ban. tie mutants were hypersensitive to radiation, and radiation sensitivity of tie mutants was rescued by increased ban gene dosage. We propose that dying cells activate ban in surviving cells through Tie to make the latter cells harder to kill, thereby preserving tissues and ensuring organism survival. The protective effect we report differs from classical radiation bystander effect in which neighbors of irradiated cells become more prone to death. The protective effect also differs from the previously described effect of dying cells that results in proliferation of nearby cells in Drosophila larval discs. If conserved in mammals, a phenomenon in which dying cells make the rest harder to kill by IR could have implications for treatments that involve the sequential use of cytotoxic agents and radiation therapy.