Phosphorylation of the novel mTOR substrate Unkempt regulates cellular morphogenesis

Mechanistic target of rapamycin (mTOR) is a protein kinase that integrates multiple inputs to regulate anabolic cellular processes. mTOR has key functions in nervous system development and mis-regulation of mTOR signalling causes aberrant neurodevelopment and results in neurological disease. Very little is understood about the signalling components that act downstream of mTOR to regulate neuronal development and morphogenesis. Here we show that the RNA-binding protein Unkempt, which is primarily expressed in the nervous system, is a novel substrate of mTOR complex I (mTORC1). Unkempt phosphorylation is regulated by nutrient levels and growth factors via mTORC1. Unkempt physically interacts with and is directly phosphorylated by mTORC1 through binding to the regulatory-associated protein of mTOR, Raptor. Phosphorylation of Unkempt during nervous system development is mTORC1-dependent, suggesting that Unkempt is a key mTORC1 substrate in the developing brain. Unkempt phosphorylation by mTORC1 occurs largely within a highly serine-rich intrinsically disordered region. Mutation analysis of key serine/threonine residues in the serine-rich region indicates that phosphorylation inhibits the ability of Unkempt to induce early bipolar neuronal-like morphology. Phosphorylation within this serine-rich region thus profoundly affects the ability of Unkempt to regulate cellular morphogenesis. Our findings reveal a molecular link between mTOR signalling and cellular morphogenesis and shed new light on this critical aspect of nervous system development. ### Competing Interest Statement The authors have declared no competing interest.