Modulation of TORC2 Signaling by a Conserved Lkb1 Signaling Axis in Budding Yeast

Abstract

Nutrient availability, growth rate and cell size are closely linked. For example, in budding yeast, the rate of cell growth is proportional to nutrient availability, cell size is proportional to growth rate, and growth rate is proportional to cell size. Thus, cells grow slowly in poor nutrients and are nearly half the size of cells growing in rich nutrients. Moreover, large cells grow faster than small cells. A signaling network that surrounds Tor kinase complex 2 (TORC2) plays an important role in enforcing these proportional relationships. Cells that lack components of the TORC2 network fail to modulate their growth rate or size in response to changes in nutrient availability. Here, we show that budding yeast homologs of the Lkb1 tumor suppressor kinase are required for normal modulation of TORC2 signaling in response to changes in carbon source. Lkb1 kinases activate Snf1/AMPK to initiate transcription of genes required for utilization of poor carbon sources. However, Lkb1 influences TORC2 signaling via a novel pathway that is independent of Snf1/AMPK. Of the three Lkb1 homologs in budding yeast, Elm1 plays the most important role in modulating TORC2. Elm1 activates a pair of related kinases called Gin4 and Hsl1. Previous work found that loss of Gin4 and Hsl1 causes cells to undergo unrestrained growth during a prolonged mitotic arrest, which suggests that they play a role in linking cell cycle progression to cell growth. We found that Gin4 and Hsl1 also control the TORC2 network. In addition, Gin4 and Hsl1 are themselves influenced by signals from the TORC2 network, consistent with previous work showing that the TORC2 network constitutes a feedback loop. Together, the data suggest a model in which the TORC2 network sets growth rate in response to carbon source, while also relaying signals via Gin4 and Hsl1 that set the critical amount of growth required for cell cycle progression. This kind of close linkage between control of cell growth and size would suggest a simple mechanistic explanation for the proportional relationship between cell size and growth rate.3

Conserved Ark1-related kinases function in a TORC2 signaling network

“…Here, we found that hyperphosphorylation of Rts1 in response to poor carbon was strongly reduced in ark1∆ prk1∆ cells ( Figure 4D ). The effects of ark1∆ prk1∆ on Rts1 hyperphosphorylation are similar to the effects caused by inactivation of Ypk1/2 or other components of the TORC2 network ( Alcaide-Gavilan et al , 2018 ).…”

“…In previous work, we found that Rts1 undergoes TORC2-dependent hyperphosphorylation in response to a shift to poor carbon ( Alcaide-Gavilan et al , 2018 ). Here, we found that hyperphosphorylation of Rts1 in response to poor carbon was strongly reduced in ark1∆ prk1∆ cells ( Figure 4D ).…”

“…Ypk1/2 control production of ceramide in two ways: they stimulate production of sphingolipids that serve as precursors for ceramide synthesis, and they directly activate ceramide synthase, which builds ceramide from the sphingolipid precursors. The TORC2 signaling network includes a poorly understood negative feedback loop in which ceramide-dependent signals appear to inhibit TORC2 signaling ( Roelants et al , 2011 ; Berchtold et al , 2012 ; Alcaide-Gavilan et al , 2018 ; Lucena et al , 2018 ). The TORC2 network may also respond to changes in membrane tension ( Riggi et al , 2018 ).…”

“…Recent work has shown that the TORC2 network relays nutrient-dependent signals that influence cell size and growth rate ( Alcaide-Gavilan et al , 2018 ; Lucena et al , 2018 ). In cells that span all orders of life, the growth rate of proliferating cells is proportional to nutrient availability, and cell size is proportional to growth rate ( Schaechter et al , 1958 ; Hirsch and Han, 1969 ; Johnston et al , 1977 ; Ferrezuelo et al , 2012 ; Leitao and Kellogg, 2017 ).…”

Growth-dependent activation of protein kinases suggests a mechanism for measuring cell growth

“…Gin4-related kinases are required for organization of septins at the bud neck, and for controlling the pattern and location of growth. Gin4-related kinases also appear to be embedded in the TORC2 network, which influences cell size and growth rate, although the functional relationships between the TORC2 network and Gin4-related kinases are poorly understood (Alcaide-Gavilan et al 2018). In each case, Gin4-related kinases appear to be closely associated with signals that influence the location, pattern and extent of growth.…”