Scaling of subcellular actin structures with cell length through decelerated growth

Improved tools for live imaging of F-actin structures in yeast

For over 20 years, the most effective probe for live imaging of yeast actin cables has been Abp140-GFP. Here, we report that endogenously-tagged Abp140-GFP poorly decorates actin patches and cables in the bud compartment of yeast cells, while robustly decorating these structures in the mother cell. Using mutagenesis, we found that asymmetric decoration by Abp140 requires F-actin binding. By expressing integrated Bni1-Bnr1 and Bnr1-Bni1 chimeras, we demonstrate that asymmetric cable decoration by Abp140 also does not depend on which formin assembles the cables in each compartment. In contrast, the short actin-binding fragment of Abp140 (known as "Lifeact"), fused to 1x or 3xmNeonGreen and expressed from the endogenous ABP140 promoter, uniformly decorates patches and cables in both compartments. Further, this probe dramatically improves live imaging detection of cables (and patches) without altering their in vivo dynamics or cell growth. Improved detection allows us to visualize cables growing inward from the cell cortex and dynamically interacting with the vacuole. This probe also robustly decorates the cytokinetic actomyosin ring. Because Lifeact-3xmNeon expressed at relatively low levels provides intense labeling of cellular F-actin structures, this tool may improve live imaging in other organisms where higher levels of Lifeact expression are detrimental.

Length control emerges from cytoskeletal network geometry

Many cytoskeletal networks consist of individual filaments that are organized into elaborate higher-order structures. While it is appreciated that the size and architecture of these networks are critical for their biological functions, much of the work investigating control over their assembly has focused on mechanisms that regulate the turnover of individual filaments through size-dependent feedback. Here, we propose a very different, feedback-independent mechanism to explain how yeast cells control the length of their actin cables. Our findings, supported by quantitative cell imaging and mathematical modeling, indicate that actin cable length control is an emergent property that arises from the cross-linked and bundled organization of the filaments within the cable. Using this model, we further dissect the mechanisms that allow cables to grow longer in larger cells and propose that cell length-dependent tuning of formin activity allows cells to scale cable length with cell length. This mechanism is a significant departure from prior models of cytoskeletal filament length control and presents a different paradigm to consider how cells control the size, shape, and dynamics of higher-order cytoskeletal structures.

Cell-size-dependent regulation of Ezrin dictates epithelial resilience to stretch by countering myosin-II-mediated contractility

The epithelial adaptations to mechanical stress are facilitated by molecular and tissue-scale changes that include the strengthening of junctions, cytoskeletal reorganization, and cell-proliferation-mediated changes in tissue rheology. However, the role of cell size in controlling these properties remains underexplored. Our experiments in the zebrafish embryonic epidermis, guided by theoretical estimations, reveal a link between epithelial mechanics and cell size, demonstrating that an increase in cell size compromises the tissue fracture strength and compliance. We show that an increase in E-cadherin levels in the proliferation-deficient epidermis restores epidermal compliance but not the fracture strength, which is largely regulated by Ezrin-an apical membrane-cytoskeleton crosslinker. We show that Ezrin fortifies the epithelium in a cell-size-dependent manner by countering non-muscle myosin-II-mediated contractility. This work uncovers the importance of cell size maintenance in regulating the mechanical properties of the epithelium and fostering protection against future mechanical stresses.

Functional redundancy and formin-independent localization of tropomyosin isoforms in Saccharomyces cerevisiae

Tropomyosin is an actin binding protein which protects actin filaments from cofilin-mediated disassembly. Distinct tropomyosin isoforms have long been hypothesized to differentially sort to subcellular actin networks and impart distinct functionalities. Nevertheless, a mechanistic understanding of the interplay between Tpm isoforms and their functional contributions to actin dynamics has been lacking. In this study, we present acetylation-mimic engineered mNeonGreen-Tpm fusion proteins that exhibit complete functionality as a sole copy, surpassing limitations of existing probes and enabling real-time dynamic tracking of Tpm-actin filaments in vivo. Using these functional Tpm fusion proteins, we find that both Tpm1 and Tpm2 indiscriminately bind to actin filaments nucleated by either formin isoform- Bnr1 and Bni1 in vivo, in contrast to the long-held paradigm of Tpm-formin pairing. We also show that Tpm2 can protect and organize functional actin cables in absence of Tpm1. Overall, our work supports a concentration-dependent and formin-independent model of Tpm-actin binding and demonstrates for the first time, the functional redundancy of the paralog Tpm2 in actin cable maintenance in S. cerevisiae.

IntAct: A nondisruptive internal tagging strategy to study the organization and function of actin isoforms

Mammals have 6 highly conserved actin isoforms with nonredundant biological functions. The molecular basis of isoform specificity, however, remains elusive due to a lack of tools. Here, we describe the development of IntAct, an internal tagging strategy to study actin isoforms in fixed and living cells. We identified a residue pair in β-actin that permits tag integration and used knock-in cell lines to demonstrate that IntAct β-actin expression and filament incorporation is indistinguishable from wild type. Furthermore, IntAct β-actin remains associated with common actin-binding proteins (ABPs) and can be targeted in living cells. We demonstrate the usability of IntAct for actin isoform investigations by showing that actin isoform-specific distribution is maintained in human cells. Lastly, we observed a variant-dependent incorporation of tagged actin variants into yeast actin patches, cables, and cytokinetic rings demonstrating cross species applicability. Together, our data indicate that IntAct is a versatile tool to study actin isoform localization, dynamics, and molecular interactions.

Length control emerges from cytoskeletal network geometry

Many cytoskeletal networks consist of individual filaments that are organized into elaborate higher order structures. While it is appreciated that the size and architecture of these networks are critical for their biological functions, much of the work investigating control over their assembly has focused on mechanisms that regulate the turnover of individual filaments through size-dependent feedback. Here, we propose a very different, feedback-independent mechanism to explain how yeast cells control the length of their actin cables. Our findings, supported by quantitative cell imaging and mathematical modeling, indicate that actin cable length control is an emergent property that arises from the cross-linked and bundled organization of the filaments within the cable. Using this model, we further dissect the mechanisms that allow cables to grow longer in larger cells, and propose that cell length-dependent tuning of formin activity allows cells to scale cable length with cell length. This mechanism is a significant departure from prior models of cytoskeletal filament length control and presents a new paradigm to consider how cells control the size, shape, and dynamics of higher order cytoskeletal structures.

Evolutionary tuning of barbed end competition allows simultaneous construction of architecturally distinct actin structures

How cells simultaneously assemble actin structures of distinct sizes, shapes, and filamentous architectures is still not well understood. Here, we used budding yeast as a model to investigate how competition for the barbed ends of actin filaments might influence this process. We found that while vertebrate capping protein (CapZ) and formins can simultaneously associate with barbed ends and catalyze each other's displacement, yeast capping protein (Cap1/2) poorly displaces both yeast and vertebrate formins. Consistent with these biochemical differences, in vivo formin-mediated actin cable assembly was strongly attenuated by the overexpression of CapZ but not Cap1/2. Multiwavelength live cell imaging further revealed that actin patches in cap2∆ cells acquire cable-like features over time, including recruitment of formins and tropomyosin. Together, our results suggest that the activities of S. cerevisiae Cap1/2 have been tuned across evolution to allow robust cable assembly by formins in the presence of high cytosolic levels of Cap1/2, which conversely limit patch growth and shield patches from formins.

Regulation of organelle size and organization during development

During early embryogenesis, as cells divide in the developing embryo, the size of intracellular organelles generally decreases to scale with the decrease in overall cell size. Organelle size scaling is thought to be important to establish and maintain proper cellular function, and defective scaling may lead to impaired development and disease. However, how the cell regulates organelle size and organization are largely unanswered questions. In this review, we summarize the process of size scaling at both the cell and organelle levels and discuss recently discovered mechanisms that regulate this process during early embryogenesis. In addition, we describe how some recently developed techniques and Xenopus as an animal model can be used to investigate the underlying mechanisms of size regulation and to uncover the significance of proper organelle size scaling and organization.

mNG-tagged fusion proteins and nanobodies to visualize tropomyosins in yeast and mammalian cells

Tropomyosins are structurally conserved α-helical coiled-coil proteins that bind along the length of filamentous actin (F-actin) in fungi and animals. Tropomyosins play essential roles in the stability of actin filaments and in regulating myosin II contractility. Despite the crucial role of tropomyosin in actin cytoskeletal regulation, in vivo investigations of tropomyosin are limited, mainly due to the suboptimal live-cell imaging tools currently available. Here, we report on an mNeonGreen (mNG)-tagged tropomyosin, with native promoter and linker length configuration, that clearly reports tropomyosin dynamics in Schizosaccharomyces pombe (Cdc8), Schizosaccharomyces japonicus (Cdc8) and Saccharomyces cerevisiae (Tpm1 and Tpm2). We also describe a fluorescent probe to visualize mammalian tropomyosin (TPM2 isoform). Finally, we generated a camelid nanobody against S. pombe Cdc8, which mimics the localization of mNG-Cdc8 in vivo. Using these tools, we report the presence of tropomyosin in previously unappreciated patch-like structures in fission and budding yeasts, show flow of tropomyosin (F-actin) cables to the cytokinetic actomyosin ring and identify rearrangements of the actin cytoskeleton during mating. These powerful tools and strategies will aid better analyses of tropomyosin and F-actin cables in vivo.

Quantitative Analysis of Actin Cable Length in Yeast

Polarized actin cables in S. cerevisiae are linear bundles of crosslinked actin filaments that are assembled by two formins, Bnr1 (localized to the bud neck), and Bni1 (localized to the bud tip). Actin is polymerized at these two sites, which results in cables extending along the cell cortex toward the back of the mother cell. These cables serve as polarized tracks for myosin-based transport of secretory vesicles and other cargo, from the mother cell to the growing daughter cell. Until recently, descriptions of actin cable morphology and architecture have largely been qualitative or descriptive in nature. Here, we introduce a new quantitative method that enables more precise characterization of actin cable length. This technological advance generates quantitative datasets that can be used to determine the contributions of different actin regulatory proteins to the maintenance of cable architecture, and to assess how different pharmacological agents affect cable arrays. Additionally, these datasets can be used to test theoretical models, and be compared to results from computational simulations of actin assembly. Graphical abstract: Illustration of actin cable length and morphology analysis. (A) Representative maximum intensity projection image of S. cerevisiae fixed and stained with fluorescently-conjugated phalloidin to label F-actin (displayed in color), and fluorescently-conjugated Concanavalin A to label the cell wall (displayed in grey scale). Lengths of actin cables traced from the bud neck to their ends are indicated (dashed lines). (B) Inverted grey scale image of F-actin labelled with fluorescently-conjugated phalloidin and the cell wall traced in black. The length (purple) and end-to-end distance (green) of a single actin cable is indicated. Scale bar, 2 µm. (C-E) Actin cable length (C), end-to-end distance (D), and tortuosity (E) from hypothetical datasets, where each data point represents an individual cable and larger symbols represent the mean from each hypothetical experiment. Error bars, 95% confidence intervals.

How to assemble a scale-invariant gradient

Intracellular protein gradients serve a variety of functions, such as the establishment of cell polarity or to provide positional information for gene expression in developing embryos. Given that cell size in a population can vary considerably, for the protein gradients to work properly they often have to be scaled to the size of the cell. Here, we examine a model of protein gradient formation within a cell that relies on cytoplasmic diffusion and cortical transport of proteins toward a cell pole. We show that the shape of the protein gradient is determined solely by the cell geometry. Furthermore, we show that the length scale over which the protein concentration in the gradient varies is determined by the linear dimensions of the cell, independent of the diffusion constant or the transport speed. This gradient provides scale-invariant positional information within a cell, which can be used for assembly of intracellular structures whose size is scaled to the linear dimensions of the cell, such as the cytokinetic ring and actin cables in budding yeast cells.

Scaling of subcellular actin structures with cell length through decelerated growth

How cells tune the size of their subcellular parts to scale with cell size is a fundamental question in cell biology. Until now, most studies on the size control of organelles and other subcellular structures have focused on scaling relationships with cell volume, which can be explained by limiting pool mechanisms. Here, we uncover a distinct scaling relationship with cell length rather than volume, revealed by mathematical modeling and quantitative imaging of yeast actin cables. The extension rate of cables decelerates as they approach the rear of the cell, until cable length matches cell length. Further, the deceleration rate scales with cell length. These observations are quantitatively explained by a ‘balance-point’ model, which stands in contrast to limiting pool mechanisms, and describes a distinct mode of self-assembly that senses the linear dimensions of the cell.