Raptor, a positive regulatory subunit of mTOR complex 1, is a novel phosphoprotein of the rDNA transcription machinery in nucleoli and chromosomal nucleolus organizer regions (NORs)

Abstract

Raptor is the key scaffolding protein that recruits mTOR substrates to rapamycin-sensitive mTOR complex 1 (mTORC1), a molecular integrator of mitogenic and nutrient/energy environmental inputs into protein translation and cell growth. Although Raptor phosphorylation on various sites is pivotal in the regulation of mTORC1 activity, it remains to be elucidated whether site-specific phosphorylation differentially distributes Raptor to unique subcellular compartments. When exploring the spatio-temporal cell cycle-dynamics of six different phospho (P)-Raptor isoforms (thr706, Ser722, Ser863, Ser792 and Ser877), a number of remarkable events differentially defined a topological resetting of P-RaptorThr706 on interphasic and mitotic chromosomes. In interphase nuclei, P-RaptorThr706 co-localized with fibrillarin, a component of the nucleolar small nuclear ribonucleoprotein particle, as well as with RNA polymerase I, the enzyme that transcribes nucleolar rRNA. Upon actinomycin D-induced nucleolar segregation and disaggregation, P-RaptorThr706 was excluded from nucleolus to accumulate at discrete nucleoplasmic bodies. During mitosis, CDK1 inhibition-induced premature assembly of nucleoli relocated fibrillarin to the surrounding regions of chromosomal-associated P-Raptor Thr706, suggesting that a sub-population of mitotic P-Raptor Thr706 remained targeted at chromosomal loops of rDNA or nuclear organizer regions (NORs). At the end of mitosis and cytokinesis, when re-assembly of incipient nucleoli begins upon NORs activation of rDNA transcription, fibrillarin spatially reorganized with P-RaptorThr706 to give rise to daughter nucleoli. treatment with IGF1 exclusively hyperactivated nuclear P-RaptorSer706 and concomitantly promoted Ser2481-autophosphorylation of mTOR, which monitors mTORC1-associated catalytic activity. Nucleolar- and NOR-associated P-RaptorSer706 may physically link mTORC1 signaling to ever-growing nucleolus plurifunctionality including ribosome biogenesis, cell stress sensor and cell cycle/aging control. © 2011 Landes Bioscience.