A MAP4 kinase related to Ste20 is a nutrient-sensitive regulator of mTOR signalling

Abstract

The mTOR (mammalian target of rapamycin) signalling pathway is a key regulator of cell growth and is controlled by growth factors and nutrients such as amino acids. Although signalling pathways from growth factor receptors to mTOR have been elucidated, the pathways mediating signalling by nutrients are poorly characterized. Through a screen for protein kinases active in the mTOR signalling pathway in Drosophila we have identified a Ste20 family member (MAP4K3) that is required for maximal S6K (S6 kinase)/4E-BP1 [eIF4E (eukaryotic initiation factor 4E)-binding protein 1] phosphorylation and regulates cell growth. Importantly, MAP4K3 activity is regulated by amino acids, but not the growth factor insulin and is not regulated by the mTORC1 inhibitor rapamycin. Our results therefore suggest a model whereby nutrients signal to mTORC1 via activation of MAP4K3.Abbreviations: DMEM, Dulbecco's modified Eagle's medium; DPBS, Dulbecco's PBS; dsRNA, double-stranded RNA; eIF4E, eukaryotic initiation factor 4E; 4E-BP1, eIF4E-binding protein; FCS, foetal calf serum; ERK, extracellular-signal-regulated kinase; GST, glutathione S-transferase; HA, haemagglutinin; HEK, human embryonic kidney; JNK, c-Jun N-terminal kinase; MAPK, mitogen-activated protein kinase; MBP, myelin basic protein; MEM, minimal essential medium; Msn, misshapen; mTOR, mammalian target of rapamycin; p90RSK, p90 ribosomal S6 kinase; PI3K, phosphoinositide 3-kinase; PKB, protein kinase B; RNAi, RNA interfering; PTEN, phosphatase and tensin homologue deleted on chromosome 10; siRNA, small interference RNA; TSC, tuberous sclerosis complex; TSC1-2, TSC1–TSC2 complex