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1. A novel interplay between GEFs orchestrates Cdc42 activity during cell polarity and cytokinesis

   https://doi.org/10.1242/jcs.236018  

   Hercyk, Brian S.; Rich-Robinson, Julie; Mitoubsi, Ahmad S.; Harrell, Marcus A.; Das, Maitreyi E. (January 2019, Journal of Cell Science)

   Cdc42, a conserved regulator of cell polarity, is activated by two GEFs, Gef1 and Scd1, in fission yeast. Why the cell needs two GEFs is unclear, given that they are partially redundant and activate the same GTPase. Using the GEF localization pattern during cytokinesis as a paradigm, we report a novel interplay between Gef1 and Scd1 that spatially modulates Cdc42. We find that Gef1 promotes Scd1 localization to the division site during cytokinesis through the recruitment of the scaffold Scd2 via a Cdc42 feedforward pathway. Similarly, in interphase Gef1 promotes Scd1 recruitment at the new end to enable the transition from monopolar to bipolar growth. Reciprocally, Scd1 restricts Gef1 localization to prevent ectopic Cdc42 activation during cytokinesis to promote cell separation, and to maintain cell shape during interphase. Our findings reveal an elegant regulatory pattern in which Gef1 primes Cdc42 activation at new sites to initiate Scd1-dependent polarized growth, while Scd1 restricts Gef1 to sites of polarization. We propose that crosstalk between GEFs is a conserved mechanism that orchestrates Cdc42 activation during complex cellular processes. 

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2. F-BAR Cdc15 Promotes Cdc42 Activation During Cytokinesis and Cell Polarization in Schizosaccharomyces pombe

   https://doi.org/10.1534/genetics.119.302649  

   Hercyk, Brian S; Das, Maitreyi E (December 2019, Genetics)

   Abstract Cdc42, a Rho-family GTPase, is a master regulator of cell polarity. Recently, it has been shown that Cdc42 also facilitates proper cytokinesis in the fission yeast Schizosaccharomyces pombe. Cdc42 is activated by two partially redundant GEFs, Gef1 and Scd1. Although both GEFs activate Cdc42, their deletion mutants display distinct phenotypes, indicating that they are differentially regulated by an unknown mechanism. During cytokinesis, Gef1 localizes to the division site and activates Cdc42 to initiate ring constriction and septum ingression. Here, we report that the F-BAR protein Cdc15 promotes Gef1 localization to its functional sites. We show that cdc15 promotes Gef1 association with cortical puncta at the incipient division site to activate Cdc42 during ring assembly. Moreover, cdc15 phospho-mutants phenocopy the polarity phenotypes of gef1 mutants. In a hypermorphic cdc15 mutant, Gef1 localizes precociously to the division site and is readily detected at the cortical patches and the cell cortex. Correspondingly, the hypermorphic cdc15 mutant shows increased bipolarity during interphase and precocious Cdc42 activation at the division site during cytokinesis. Finally, loss of gef1 in hypermorphic cdc15 mutants abrogates the increased bipolarity and precocious Cdc42 activation phenotype. We did not see any change in the localization of the other GEF Scd1 in a Cdc15-dependent manner. Our data indicate that Cdc15 facilitates Cdc42 activation at the division site during cytokinesis at the cell cortex to promote bipolarity and this is mediated by promoting Gef1 localization to these sites. 

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3. Organelles harbour pH gradients

   https://doi.org/10.21203/rs.3.rs-8395568/v1  

   Krishnan, Yamuna; Lee, Sangyoon; Chakraborty, Sandip; Kim, Soyoung; Ali, Asif; Mitra, Koushambi; Zajac, Matthew; Brown, Anastasia; Pincus, David (January 2026, Research Square)

   Abstract Organelle pH is critical to organelle identity and function. Resident proteins that define each organelle modify transiting cargo proteins, with both retention and trafficking between organelles governed by pH-dependent mechanisms. For example, lysosomal enzymes bind mannose-6-phosphate receptors at the higher pH (~6.5) of the Golgi and dissociate at the lower pH (~5.5) of late endosomes. Proteins that stray from the endoplasmic reticulum (ER) are captured by KDEL receptors in the acidic Golgi and returned into the neutral ER. This pH-tuned trafficking system compartmentalizes organelle function and prevents mis-localization of critical enzymes. Dysregulated organelle pH disrupts their function and leads to various diseases. Because protons move rapidly in water, the pH within a single organelle is currently assumed to be spatially uniform. Here, using a reporter sensitive from pH 5.5 - 10.5 to map a spectrum of organelles at high resolution, we discovered that pH gradients exist within single, large or long organelles such as the ER and mitochondria, and in membrane-less organelles without ion-transporting proteins such as the nucleolus. These new findings upend our understanding of organellar pH, prompting new questions about proton diffusion within the cell, and its potential consequences on organelle function. 

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4. WALLFLOWER, a RLK, simultaneously localizes to opposite sides of root hair cells & functions to position hairs

   https://doi.org/10.1101/2023.03.16.533027  

   Toth, Jessica N; Rodriguez-Furlan, Cecilia; Van_Norman, Jaimie M (March 2023, bioRxiv)

   ABSTRACT Polarized cells are frequently partitioned into subdomains with unique features or functions. As plant cells are surrounded by walls, polarized cell shape and protein polarity in the plasma membrane are particularly important for normal physiology and development. We have identified WALLFLOWER (WFL), a transmembrane receptor kinase that is asymmetrically distributed at the inner face of epidermal cells and this localization is maintained independent of cell type. In epidermal hair (H) cells in the elongation and differentiation zones, WFL exhibits a dual polar localization, accumulating at the inner domain as well as at the root hair initiation domain (RHID). Furthermore, overexpression of WFL leads to a downward shift in root hair (RH) position suggesting WFL operates in a signaling pathway that functions across H cells to inform RH position. WFL asymmetric distribution and function is affected by deletion of the intracellular domains resulting in its mislocalization to the outer polar domain of H cells and exclusion from RHIDs and bulges. Thus, our results demonstrate that in epidermal H cells the WFL intracellular domains are required to direct its dual polar localization and influence RH position. ONE SENTENCE SUMMARYA receptor kinase with dual polar localization, to the inner polar domain and root hair initiation domain, in root epidermal cells, requires its intracellular domain for localization and function. 

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5. Organelles harbour pH gradients

   https://doi.org/10.64898/2025.12.12.694065  

   Lee, Sangyoon; Chakraborty, Sandip; Kim, Soyoung; Ali, Asif; Mitra, Koushambi; Zajac, Matthew; Brown, Anastasia; Pincus, David; Krishnan, Yamuna (December 2025, bioRxiv/PMC)

   Organelle pH is critical to organelle identity and function. Resident proteins that define each organelle modify transiting cargo proteins, with both retention and trafficking between organelles governed by pH-dependent mechanisms. For example, lysosomal enzymes bind mannose-6-phosphate receptors at the higher pH (∼6.5) of the Golgi and dissociate at the lower pH (∼5.5) of late endosomes¹. Proteins that stray from the endoplasmic reticulum (ER) are captured by KDEL receptors in the acidic Golgi and returned into the neutral ER^2,3. This pH-tuned trafficking system compartmentalizes organelle function and prevents mis-localization of critical enzymes⁴. Dysregulated organelle pH disrupts their function and leads to various diseases. Because protons move rapidly in water, the pH within a single organelle is currently assumed to be spatially uniform⁵. Here, using a reporter sensitive from pH 5.5 - 10.5 to map a spectrum of organelles at high resolution, we discovered that pH gradients exist within single, large or long organelles such as the ER and mitochondria, and in membrane-less organelles without ion-transporting proteins such as the nucleolus. These new findings upend our understanding of organellar pH, prompting new questions about proton diffusion within the cell, and its potential consequences on organelle function. 

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