A component of the TOR (Target Of Rapamycin) nutrient-sensing pathway plays a role in circadian rhythmicity in Neurospora crassa

Abstract

The TOR (Target of Rapamycin) pathway is a highly-conserved signaling pathway in eukaryotes that regulates cellular growth and stress responses. The cellular response to amino acids or carbon sources such as glucose requires anchoring of the TOR kinase complex to the lysosomal/vacuolar membrane by the Ragulator (mammals) or EGO (yeast) protein complex. Here we report a connection between the TOR pathway and circadian (daily) rhythmicity. The molecular mechanism of circadian rhythmicity in all eukaryotes has long been thought to be transcription/translation feedback loops (TTFLs). In the model eukaryote Neurospora crassa, a TTFL including FRQ (frequency) and WCC (white collar complex) has been intensively studied. However, it is also well-known that rhythmicity can be seen in the absence of TTFL functioning. We previously isolated uv90 as a mutation that compromises FRQ-less rhythms and also damps the circadian oscillator when FRQ is present. We have now mapped the uv90 gene and identified it as NCU05950, homologous to the TOR pathway proteins EGO1 (yeast) and LAMTOR1 (mammals), and we have named the N. crassa protein VTA (vacuolar TOR-associated protein). The protein is anchored to the outer vacuolar membrane and deletion of putative acylation sites destroys this localization as well as the protein’s function in rhythmicity. A deletion of VTA is compromised in its growth responses to amino acids and glucose. We conclude that a key protein in the complex that anchors TOR to the vacuole plays a role in maintaining circadian (daily) rhythmicity. Our results establish a connection between the TOR pathway and circadian rhythms and point towards a network integrating metabolism and the circadian system.

mTOR: A Cellular Regulator Interface in Health and Disease

“…TORC1 activation is tightly coupled to diet-induced changes in amino acid concentrations, because amino acids are not only essential building blocks of proteins, but also sources of energy and carbon for many other metabolic pathways [25]. Moreover, recent work from the fungus Neurospora crassa shows that a RAGULATOR-like protein is involved in amino acid sensing, as well as in the regulation of the circadian clock, strongly suggesting a coordinated link between circadian rhythms and TOR-dependent metabolism [63].…”

Fungal plasma membrane domains

“…In both budding (Heitman, Movva and Hall 1991) and fission yeasts (Hayashi et al 2007;Ikai et al 2011), two Tor paralogs exist and populate two structurally and physiologically distinct protein complexes known as the Target Of Rapamycin Complex 1 (TORC1) and 2 (TORC2) (Loewith et al 2002;Wedaman et al 2003). In other fungal species (such as N. crassa and A. nidulans), but also in mammals, a single Tor protein exists in both complexes (Fitzgibbon et al 2005;De Souza et al 2013;Ratnayake et al 2018;Stuttfeld et al 2018). The TORC1 is highly conserved in eukaryotes (Soulard, Cohen and Hall 2009;Tatebe and Shiozaki 2017), is sensitive to rapamycin and controls translation, transcription, cell cycle regulation, ribosome biogenesis, and autophagy in response to nutrient availability [Reviewed in (Saxton and Sabatini 2017;Tatebe and Shiozaki 2017).…”

A pathway linking translation stress to checkpoint kinase 2 signaling in Neurospora crassa

“…Hence, our data indicate that TORC1 is the upstream kinase complex that activates PRD-4 in response to inhibition of protein translation. A knockout of VTA, a TORC1-associated regulatory component, was shown to dampen the FRQ protein rhythm (36), suggesting a connection with the Neurospora circadian clock.…”