Casein kinase 1 controls components of a TORC2 signaling network in budding yeast.

Tor kinases play diverse and essential roles in control of nutrient signaling and cell growth. Tor kinases are assembled into two large multiprotein complexes referred to as Tor Complex 1 and Tor Complex 2 (TORC1 and TORC2). In budding yeast, TORC2 controls a signaling network that relays signals regarding carbon source that strongly influence growth rate and cell size. However, the mechanisms that control TORC2 signaling are poorly understood. Activation of TORC2 requires Mss4, a phosphoinositol kinase that initiates assembly of a multi-protein complex at the plasma membrane that recruits and activates downstream targets of TORC2. Localization of Mss4 to the plasma membrane is controlled by phosphorylation and previous work suggested that yeast homologs of casein kinase 1γ, referred to as Yck1 and Yck2, control phosphorylation of Mss4. Here, we generated a new analog-sensitive allele of YCK2 and used it to test whether Yck1/2 influence signaling in the TORC2 network. We found that multiple components of the TORC2 network are strongly influenced by Yck1/2 signaling.

Growth-Dependent Activation of Protein Kinases Suggests a Mechanism for Measuring Cell Growth.

In all cells, progression through the cell cycle occurs only when sufficient growth has occurred. Thus, cells must translate growth into a proportional signal that can be used to measure and transmit information about growth. Previous genetic studies in budding yeast suggested that related kinases called Gin4 and Hsl1 could function in mechanisms that measure bud growth; however, interpretation of the data was complicated by the use of gene deletions that cause complex terminal phenotypes. Here, we used the first conditional alleles of Gin4 and Hsl1 to more precisely define their functions. We show that excessive bud growth during a prolonged mitotic delay is an immediate consequence of inactivating Gin4 and Hsl1 Thus, acute loss of Gin4 and Hsl1 causes cells to behave as though they cannot detect that bud growth has occurred. We further show that Gin4 and Hsl1 undergo gradual hyperphosphorylation during bud growth that is dependent upon growth and correlated with the extent of growth. Moreover, gradual hyperphosphorylation of Gin4 during bud growth requires binding to anionic phospholipids that are delivered to the growing bud. While alternative models are possible, the data suggest that signaling lipids delivered to the growing bud generate a growth-dependent signal that could be used to measure bud growth.

A Conserved PP2A Regulatory Subunit Enforces Proportional Relationships Between Cell Size and Growth Rate.

Cell size is proportional to growth rate. Thus, cells growing rapidly in rich nutrients can be nearly twice the size of cells growing slowly in poor nutrients. This proportional relationship appears to hold across all orders of life, yet the underlying mechanisms are unknown. In budding yeast, most growth occurs during mitosis, and the proportional relationship between cell size and growth rate is therefore enforced primarily by modulating growth in mitosis. When growth is slow, the duration of mitosis is increased to allow more time for growth, yet the amount of growth required to complete mitosis is reduced, which leads to the birth of small daughter cells. Previous studies have found that Rts1, a member of the conserved B56 family of protein phosphatase 2A regulatory subunits, works in a TORC2 signaling network that influences cell size and growth rate. However, it was unclear whether Rts1 influences cell growth and size in mitosis. Here, we show that Rts1 is required for the proportional relationship between cell size and growth rate during mitosis. Moreover, nutrients and Rts1 influence the duration and extent of growth in mitosis via Wee1 and Pds1/securin, two conserved regulators of mitotic progression. Together, the data are consistent with a model in which global signals that set growth rate also set the critical amount of growth required for cell cycle progression, which would provide a simple mechanistic explanation for the proportional relationship between cell size and growth rate.

Dynamic monitoring of STAT3 activation in live cells using a novel STAT3 Phospho-BRET sensor.

Phosphorylation (pY705) mediated homodimerization is a rate-limiting step controlling STAT3 key oncogenic functions making it an attractive target for drug discovery. Hence, this study reports development of a sensitive and versatile STAT3 Phospho-BRET biosensor platform technology to monitor activation dynamics of STAT3 signalling directly from live cells. Categorically, we first demonstrate that NanoLuc donor and TurboFP635 acceptor serves as an excellent BRET system over other tested fluorophores like mOrange and TagRFP, both for live cells as well as in vivo optical imaging of protein-protein interactions. Based on initial multi-parametric optimizations, our Phospho-BRET sensor developed by fusing STAT3 with NanoLuc and TurboFP at the C-terminus, successfully captured the activation kinetics of STAT3 in response to different ligands (e.g. IL6 & EGF) and across multiple cancer cell types either with or without the endogenous STAT3 pool. Perturbation in EGF-mediated STAT3 BRET activation signal upon blocking with EGFR neutralizing antibody further confirms the specificity of the sensor to judge ligand-receptor pathway dependent STAT3 activation. Finally, we determine the high-throughput compatibility of the developed biosensor by testing a few known/unknown STAT3 inhibitors in a 96- and 384-well plate format. The results from this screen revealed that drug molecules such as curcumin and niclosamide are more efficient inhibitors over known molecule like Stattic. Thus, the STAT3 Phospho-BRET sensor is a first of its kind live cell platform technology developed for its use to study STAT3 pathway dynamics and screen potential drug molecules in vivo.

Evolution of BRET Biosensors from Live Cell to Tissue-Scale In vivo Imaging.

Development of bioluminescence resonance energy transfer (BRET) based genetic sensors for sensing biological functions such as protein-protein interactions (PPIs) in vivo has a special value in measuring such dynamic events at their native environment. Since its inception in the late nineties, BRET related research has gained significant momentum in terms of adding versatility to the assay format and wider applicability where it has been suitably used. Beyond the scope of quantitative measurement of PPIs and protein dimerization, molecular imaging applications based on BRET assays have broadened its scope for screening pharmacologically important compounds by in vivo imaging as well. In this mini-review we focus on an in-depth analysis of engineered BRET systems developed and their successful application to cell-based assays as well as in vivo non-invasive imaging in live subjects.