Modeling thick filament activation suggests a molecular basis for force depression.

Multiscale models aiming to connect muscle's molecular and cellular function have been difficult to develop, in part due to a lack of self-consistent multiscale data. To address this gap, we measured the force response from single, skinned rabbit psoas muscle fibers to ramp shortenings and step stretches performed on the plateau region of the force-length relationship. We isolated myosin from the same muscles and, under similar conditions, performed single-molecule and ensemble measurements of myosin's ATP-dependent interaction with actin using laser trapping and in vitro motility assays. We fit the fiber data by developing a partial differential equation model that includes thick filament activation, whereby an increase in force on the thick filament pulls myosin out of an inhibited state. The model also includes a series elastic element and a parallel elastic element. This parallel elastic element models a titin-actin interaction proposed to account for the increase in isometric force after stretch (residual force enhancement). By optimizing the model fit to a subset of our fiber measurements, we specified seven unknown parameters. The model then successfully predicted the remainder of our fiber measurements and also our molecular measurements from the laser trap and in vitro motility. The success of the model suggests that our multiscale data are self-consistent and can serve as a testbed for other multiscale models. Moreover, the model captures the decrease in isometric force observed in our muscle fibers after active shortening (force depression), suggesting a molecular mechanism for force depression, whereby a parallel elastic element combines with thick filament activation to decrease the number of cycling cross-bridges.

Multiscale biophysical models of cardiomyopathies reveal complexities challenging existing dogmas.

Mutations in sarcomeric proteins, including myosin, cause a variety of cardiomyopathies. A prominent hypothesis has been that myosin mutations causing hypercontractility of the motor lead to hypertrophic cardiomyopathy, while those causing hypocontractility lead to dilated cardiomyopathy; however, recent biophysical studies using multiscale computational and experimental models have revealed complexities not captured by this hypothesis. We summarize recent publications in Biophysical Journal challenging this dogma and highlighting the need for multiscale modeling of these complex diseases.

Single-molecule mechanics and kinetics of cardiac myosin interacting with regulated thin filaments.

The cardiac cycle is a tightly regulated process wherein the heart generates force to pump blood to the body during systole and then relaxes during diastole. Disruption of this finely tuned cycle can lead to a range of diseases including cardiomyopathies and heart failure. Cardiac contraction is driven by the molecular motor myosin, which pulls regulated thin filaments in a calcium-dependent manner. In some muscle and nonmuscle myosins, regulatory proteins on actin tune the kinetics, mechanics, and load dependence of the myosin working stroke; however, it is not well understood whether or how thin-filament regulatory proteins tune the mechanics of the cardiac myosin motor. To address this critical gap in knowledge, we used single-molecule techniques to measure the kinetics and mechanics of the substeps of the cardiac myosin working stroke in the presence and absence of thin filament regulatory proteins. We found that regulatory proteins gate the calcium-dependent interactions between myosin and the thin filament. At physiologically relevant ATP concentrations, cardiac myosin's mechanics and unloaded kinetics are not affected by thin-filament regulatory proteins. We also measured the load-dependent kinetics of cardiac myosin at physiologically relevant ATP concentrations using an isometric optical clamp, and we found that thin-filament regulatory proteins do not affect either the identity or magnitude of myosin's primary load-dependent transition. Interestingly, at low ATP concentrations at both saturating and physiologically relevant subsaturating calcium concentrations, thin-filament regulatory proteins have a small effect on actomyosin dissociation kinetics, suggesting a mechanism beyond simple steric blocking. These results have important implications for the modeling of cardiac physiology and diseases.

FRET and optical trapping reveal mechanisms of actin activation of the power stroke and phosphate release in myosin V.

Myosins generate force and motion by precisely coordinating their mechanical and chemical cycles, but the nature and timing of this coordination remains controversial. We utilized a FRET approach to examine the kinetics of structural changes in the force-generating lever arm in myosin V. We directly compared the FRET results with single-molecule mechanical events examined by optical trapping. We introduced a mutation (S217A) in the conserved switch I region of the active site to examine how myosin couples structural changes in the actin- and nucleotide-binding regions with force generation. Specifically, S217A enhanced the maximum rate of lever arm priming (recovery stroke) while slowing ATP hydrolysis, demonstrating that it uncouples these two steps. We determined that the mutation dramatically slows both actin-induced rotation of the lever arm (power stroke) and phosphate release (≥10-fold), whereas our simulations suggest that the maximum rate of both steps is unchanged by the mutation. Time-resolved FRET revealed that the structure of the pre- and post-power stroke conformations and mole fractions of these conformations were not altered by the mutation. Optical trapping results demonstrated that S217A does not dramatically alter unitary displacements or slow the working stroke rate constant, consistent with the mutation disrupting an actin-induced conformational change prior to the power stroke. We propose that communication between the actin- and nucleotide-binding regions of myosin assures a proper actin-binding interface and active site have formed before producing a power stroke. Variability in this coupling is likely crucial for mediating motor-based functions such as muscle contraction and intracellular transport.

Positional Isomers of a Non-Nucleoside Substrate Differentially Affect Myosin Function.

Molecular motors have evolved to transduce chemical energy from ATP into mechanical work to drive essential cellular processes, from muscle contraction to vesicular transport. Dysfunction of these motors is a root cause of many pathologies necessitating the need for intrinsic control over molecular motor function. Herein, we demonstrate that positional isomerism can be used as a simple and powerful tool to control the molecular motor of muscle, myosin. Using three isomers of a synthetic non-nucleoside triphosphate, we demonstrate that myosin's force- and motion-generating capacity can be dramatically altered at both the ensemble and single-molecule levels. By correlating our experimental results with computation, we show that each isomer exerts intrinsic control by affecting distinct steps in myosin's mechanochemical cycle. Our studies demonstrate that subtle variations in the structure of an abiotic energy source can be used to control the force and motility of myosin without altering myosin's structure.

Distances and directions: An emotional journey into the recovery process.

Positive emotions stemming from leisure activities are often promoted as a way to achieve a state of recovery, in particular by counteracting negative emotions experienced throughout the workday. Yet the recovery literature frequently takes an undifferentiated view of both the positive emotions employees experience as well as the negative emotions employees are recovering from. This implicitly assumes that all positive emotions are equally effective in facilitating recovery from all negative emotions. Drawing from theory treating emotional movements as a metaphorical journey, we develop a framework for understanding recovery that highlights the importance of the distance and direction that individuals "travel" when moving from negative emotions to positive emotions during the recovery process. We argue that the negative emotions that people start with from work-that is, their emotional origin-as well as the positive emotions that people end with following leisure activities-that is, their emotional destination-jointly influence the state of being recovered. Across two studies using experience-sampling methodologies, we find that "shorter" journeys consisting of emotional destinations that match the activation level of emotional origins (e.g., experiencing high activation positive emotion [HAP] to counter high activation negative emotion) are effective in promoting recovery, while "longer" journeys consisting of mismatches (e.g., experiencing HAP to counter low activation negative emotion) are ineffective for recovery. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Comprehensive analysis reveals dynamic and evolutionary plasticity of Rab GTPases and membrane traffic in Tetrahymena thermophila.

Cellular sophistication is not exclusive to multicellular organisms, and unicellular eukaryotes can resemble differentiated animal cells in their complex network of membrane-bound structures. These comparisons can be illuminated by genome-wide surveys of key gene families. We report a systematic analysis of Rabs in a complex unicellular Ciliate, including gene prediction and phylogenetic clustering, expression profiling based on public data, and Green Fluorescent Protein (GFP) tagging. Rabs are monomeric GTPases that regulate membrane traffic. Because Rabs act as compartment-specific determinants, the number of Rabs in an organism reflects intracellular complexity. The Tetrahymena Rab family is similar in size to that in humans and includes both expansions in conserved Rab clades as well as many divergent Rabs. Importantly, more than 90% of Rabs are expressed concurrently in growing cells, while only a small subset appears specialized for other conditions. By localizing most Rabs in living cells, we could assign the majority to specific compartments. These results validated most phylogenetic assignments, but also indicated that some sequence-conserved Rabs were co-opted for novel functions. Our survey uncovered a rare example of a nuclear Rab and substantiated the existence of a previously unrecognized core Rab clade in eukaryotes. Strikingly, several functionally conserved pathways or structures were found to be associated entirely with divergent Rabs. These pathways may have permitted rapid evolution of the associated Rabs or may have arisen independently in diverse lineages and then converged. Thus, characterizing entire gene families can provide insight into the evolutionary flexibility of fundamental cellular pathways.

Stopping surface-acting spillover: A transactional theory of stress perspective.

Faking one's emotional display to fit situational norms, otherwise known as surface acting, has long been regarded as harmful to employees at work. A nascent body of literature has begun to examine the detriments of surface acting beyond the workplace, particularly as they spill over into the homelife. We articulate how the transactional theory of stress serves as a unifying framework that not only explains why surface acting tends to deplete employees and leads to maladaptive responses at home but also what coping strategies can be utilized to halt this spillover process. Using two complementary experience-sampling methodology studies of employee-spouse dyads, we test a spillover model of surface acting. In Study 1 and our Supplement to Study 1, we find support for the buffering role of challenge appraisals on the relationship between surface acting and depletion at work. We also find support for the mitigating role of supportive spousal interactions on the relationship between depletion at work and perceived inauthenticity at home. We also find support for the conditional indirect effect from surface acting at work to relationship satisfaction at home. In Study 2, we develop daily writing exercises to enhance challenge appraisals and supportive spousal interactions, and we find that these interventions also buffer the surface-acting spillover process. Overall, this work demonstrates the importance of both depletion and perceived inauthenticity at home as linked spillover mechanisms and reveals the success of agentic coping mechanisms (in both domains) that can buffer this process. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

A mutation in switch I alters the load-dependent kinetics of myosin Va.

Myosin Va is the molecular motor that drives intracellular vesicular transport, powered by the transduction of chemical energy from ATP into mechanical work. The coupling of the powerstroke and phosphate (Pi) release is key to understanding the transduction process, and crucial details of this process remain unclear. Therefore, we determined the effect of elevated Pi on the force-generating capacity of a mini-ensemble of myosin Va S1 (WT) in a laser trap assay. By increasing the stiffness of the laser trap we determined the effect of increasing resistive loads on the rate of Pi-induced detachment from actin, and quantified this effect using the Bell approximation. We observed that WT myosin generated higher forces and larger displacements at the higher laser trap stiffnesses in the presence of 30 mM Pi, but binding event lifetimes decreased dramatically, which is most consistent with the powerstroke preceding the release of Pi from the active site. Repeating these experiments using a construct with a mutation in switch I of the active site (S217A) caused a seven-fold increase in the load-dependence of the Pi-induced detachment rate, suggesting that the S217A region of switch I may help mediate the load-dependence of Pi-rebinding.

Single Molecule Mechanics and Kinetics of Cardiac Myosin Interacting with Regulated Thin Filaments.

The cardiac cycle is a tightly regulated process wherein the heart generates force to pump blood to the body during systole and then relaxes during diastole. Disruption of this finely tuned cycle can lead to a range of diseases including cardiomyopathies and heart failure. Cardiac contraction is driven by the molecular motor myosin, which pulls regulated thin filaments in a calcium-dependent manner. In some muscle and non-muscle myosins, regulatory proteins on actin tune the kinetics, mechanics, and load dependence of the myosin working stroke; however, it is not well understood whether or how thin filament regulatory proteins tune the mechanics of the cardiac myosin motor. To address this critical gap in knowledge, we used single-molecule techniques to measure the kinetics and mechanics of the substeps of the cardiac myosin working stroke in the presence and absence of thin filament regulatory proteins. We found that regulatory proteins gate the calcium-dependent interactions between myosin and the thin filament. At physiologically relevant ATP concentrations, cardiac myosin's mechanics and unloaded kinetics are not affected by thin filament regulatory proteins. We also measured the load-dependent kinetics of cardiac myosin at physiologically relevant ATP concentrations using an isometric optical clamp, and we found that thin filament regulatory proteins do not affect either the identity or magnitude of myosin's primary load-dependent transition. Interestingly, at low ATP concentrations, thin filament regulatory proteins have a small effect on actomyosin dissociation kinetics, suggesting a mechanism beyond simple steric blocking. These results have important implications for both disease modeling and computational models of muscle contraction. Significance Statement: Human heart contraction is powered by the molecular motor ß-cardiac myosin, which pulls on thin filaments consisting of actin and the regulatory proteins troponin and tropomyosin. In some muscle and non-muscle systems, these regulatory proteins tune the kinetics, mechanics, and load dependence of the myosin working stroke. Despite having a central role in health and disease, it is not well understood whether the mechanics or kinetics of ß-cardiac myosin are affected by regulatory proteins. We show that regulatory proteins do not affect the mechanics or load-dependent kinetics of the working stroke at physiologically relevant ATP concentrations; however, they can affect the kinetics at low ATP concentrations, suggesting a mechanism beyond simple steric blocking. This has important implications for modeling of cardiac physiology and diseases.

Modeling Thick Filament Activation Suggests a Molecular Basis for Force Depression.

Multiscale models aiming to connect muscle's molecular and cellular function have been difficult to develop, in part, due to a lack of self-consistent multiscale data. To address this gap, we measured the force response from single skinned rabbit psoas muscle fibers to ramp shortenings and step stretches performed on the plateau region of the force-length relationship. We isolated myosin from the same muscles and, under similar conditions, performed single molecule and ensemble measurements of myosin's ATP-dependent interaction with actin using laser trapping and in vitro motility assays. We fit the fiber data by developing a partial differential equation model that includes thick filament activation, whereby an increase in force on the thick filament pulls myosin out of an inhibited state. The model also includes a series elastic element and a parallel elastic element. This parallel elastic element models a titin-actin interaction proposed to account for the increase in isometric force following stretch (residual force enhancement). By optimizing the model fit to a subset of our fiber measurements, we specified seven unknown parameters. The model then successfully predicted the remainder of our fiber measurements and also our molecular measurements from the laser trap and in vitro motility. The success of the model suggests that our multiscale data are self-consistent and can serve as a testbed for other multiscale models. Moreover, the model captures the decrease in isometric force observed in our muscle fibers after active shortening (force depression), suggesting a molecular mechanism for force depression, whereby a parallel elastic element combines with thick filament activation to decrease the number of cycling cross-bridges.

Myosin's powerstroke occurs prior to the release of phosphate from the active site.

Myosins are a family of motor proteins responsible for various forms of cellular motility, including muscle contraction and vesicular transport. The most fundamental aspect of myosin is its ability to transduce the chemical energy from the hydrolysis of ATP into mechanical work, in the form of force and/or motion. A key unanswered question of the transduction process is the timing of the force-generating powerstroke relative to the release of phosphate (Pi ) from the active site. We examined the ability of single-headed myosin Va to generate a powerstroke in a single molecule laser trap assay while maintaining Pi in its active site, by either elevating Pi in solution or by introducing a mutation in myosin's active site (S217A) to slow Pi -release from the active site. Upon binding to the actin filament, WT myosin generated a powerstoke rapidly (≥500 s-1 ) and without a detectable delay, both in the absence and presence of 30 mM Pi . The elevated levels of Pi did, however, affect event lifetime, eliminating the longest 25% of binding events, confirming that Pi rebound to myosin's active site and accelerated detachment. The S217A construct also generated a powerstroke similar in size and rate upon binding to actin despite the slower Pi release rate. These findings provide direct evidence that myosin Va generates a powerstroke with Pi still in its active site. Therefore, the findings are most consistent with a model in which the powerstroke occurs prior to the release of Pi from the active site.

Exchanging one uncertainty for another: Justice variability negates the benefits of justice.

Although the importance of organizational justice is without question, our theoretical and empirical knowledge of the justice phenomenon is focused almost exclusively on mean levels of fair treatment, ignoring whether those mean levels are achieved in a consistent or inconsistent manner. This exclusive focus on average levels of justice is not surprising given the implicit assumption in the justice literature that day-to-day variations in justice are glossed over or reinterpreted by individuals. Building upon recent research demonstrating that variability in justice can be as important as average levels of fair treatment, we leverage tenets of uncertainty management theory to provide a conceptual bridge that integrates justice variability into the group engagement model. Our theoretical model proposes justice variability (arising from fluctuations in one's fair treatment over time) negates the very benefits that average levels of interpersonal justice provide. Results of 2, week-long experience sampling studies (one of 111 employees and one of 352 employees nested in 104 groups), used to construct assessments of day-to-day justice variability, largely supported our predictions regarding interactive effects between average levels of justice and justice variability on judgments of pride in the group and, ultimately, cooperative behavior, providing important takeaways for theory, research, and practice. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Not all fairness is created equal: A study of employee attributions of supervisor justice motives.

A large body of research demonstrates that employee perceptions of fair treatment matter. The overwhelming focus of these investigations has been on how employees react to whether or not they perceive their supervisor behaved in a fair manner. We contend, however, that employees not only question and react to whether they are treated fairly, but also to why they believe their supervisor acted fairly in the first place. To do so, we consider how employee attributions of supervisor motives for fair treatment influence the cognitive and affective mechanisms by which fair treatment influences employee reactions to fairness. Drawing from the justice actor model, we focus on both cognitive (establishing fairness, identity maintenance, and effecting compliance) and affective (positive affect) motives underlying supervisors' fair treatment. Relying on theory and research on motive attribution and leader affect, we develop predictions for how employees' perceptions of these motives as a result of short-term exchanges over time influence supervisor-directed citizenship behavior through both cognitive (trust in the supervisor) and affective (positive affect) mechanisms. Our experience sampling study of 613 weekly fair events (from 171 employees) largely supported our predictions, demonstrating that attribution of supervisor motives is a meaningful component of an employee's justice experience. (PsycINFO Database Record (c) 2020 APA, all rights reserved).

The popularity contest at work: who wins, why, and what do they receive?

In 2 studies, the authors investigated the popularity of employees at work. They tested a model that positioned personality in the form of core self-evaluations and situational position in the form of communication network centrality as antecedents of popularity and interpersonal citizenship and counterproductive work behaviors received from coworkers as outcomes of popularity. Data from 116 employees and 383 coworkers in Study 1 and 139 employees, their significant others, and 808 coworkers in Study 2 generally supported the model. Core self-evaluations and communication network centrality were positively related to popularity, and popular employees reported receiving more citizenship behaviors and fewer counterproductive work behaviors from their coworkers than less popular employees, even controlling for interpersonal liking. (PsycINFO Database Record (c) 2009 APA, all rights reserved).

The role of core self-evaluations in the coping process.

In 2 studies, the authors investigated whether core self-evaluations (CSE) serve as an integrative framework for understanding individual differences in coping processes. A meta-analytic review demonstrated that CSEs were associated with fewer perceived stressors, lower strain, less avoidance coping, more problem-solving coping, and were not strongly related to emotion-focused coping. Consistent with the meta-analytic results, a daily diary study demonstrated that individuals with high CSE perceived fewer stressors, experienced less strain after controlling for stressors, and engaged in less avoidance coping. However, both studies demonstrated that emotional stability was uniquely related to the stress and coping process and that emotional stability moderated the relationship between stressors and strain. The discussion focuses on the distinction between depressive self-concept represented by CSE and the anxiety and worry represented by emotional stability. (PsycINFO Database Record (c) 2009 APA, all rights reserved).

An actor-focused model of justice rule adherence and violation: the role of managerial motives and discretion.

Research on organizational justice has focused primarily on the receivers of just and unjust treatment. Little is known about why managers adhere to or violate rules of justice in the first place. The authors introduce a model for understanding justice rule adherence and violation. They identify both cognitive motives and affective motives that explain why managers adhere to and violate justice rules. They also draw distinctions among the justice rules by specifying which rules offer managers more or less discretion in their execution. They then describe how motives and discretion interact to influence justice-relevant actions. Finally, the authors incorporate managers' emotional reactions to consider how their actions may change over time. Implications of the model for theory, research, and practice are discussed.

Turning frowns (and smiles) upside down: A multilevel examination of surface acting positive and negative emotions on well-being.

The consensus in the emotional labor literature is that surface acting is "bad" for employees. However, the evidence on which this consensus is based has been derived from contexts emphasizing the display of positive emotions, such as customer service. Despite the acknowledgment that many contexts also require the display of negative emotions, scholarly work has proceeded under the assumption that surface acting is harmful regardless of the valence of the emotion being displayed. In this study, we take a hedonic approach to well-being and challenge the consensus that surface acting is bad for employees by examining its effects on changes in emotional exhaustion and job satisfaction, through changes in positive and negative affect, for both positive and negative emotional displays. Using a within-person approach, we focus on managers, whose occupation calls for displays of both positive and negative emotions. Our 3-week, experience-sampling study of 79 managers revealed that faking positive emotions decreases positive affect, which harms well-being more than authentically displaying such emotions. In contrast and counter to what the extant literature would suggest, faking negative emotions decreases negative affect and increases positive affect, which benefits well-being more than authentically displaying such emotions. We further integrate construal level theory with hedonic approaches of emotion to identify trait construal level as an important boundary condition to explain for whom surface acting is harmful versus beneficial. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Ethical leadership as a substitute for justice enactment: An information-processing perspective.

Why do employees perceive that they have been treated fairly by their supervisor? Theory and research on justice generally presumes a straightforward answer to this question: Because the supervisor adhered to justice rules. We propose the answer is not so straightforward and that employee justice perceptions are not merely "justice-laden." Drawing from theory on information processing that distinguishes between automatic and systematic modes, we suggest that employee justice perceptions are also "ethics-laden." Specifically, we posit that employees with more ethical supervisors form justice perceptions through automatic processing with little scrutiny of or attention paid to a supervisor's justice acts. In contrast, employees with less ethical supervisors rely on systematic processing to evaluate their supervisor's justice enactment and form justice perceptions. Thus, we propose that ethical leadership substitutes for the supervisor's justice enactment. Our results demonstrate support for the interactive effect of supervisor justice enactment and ethical leadership on employee justice perceptions, and we further demonstrate its consequences for employees' engagement in discretionary behaviors (citizenship and counterproductive behaviors). Our findings highlight an assumption in the justice literature in need of revision and opens the door to further inquiry about the role of information processing in justice perceptions. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Trust, trustworthiness, and trust propensity: a meta-analytic test of their unique relationships with risk taking and job performance.

The trust literature distinguishes trustworthiness (the ability, benevolence, and integrity of a trustee) and trust propensity (a dispositional willingness to rely on others) from trust (the intention to accept vulnerability to a trustee based on positive expectations of his or her actions). Although this distinction has clarified some confusion in the literature, it remains unclear (a) which trust antecedents have the strongest relationships with trust and (b) whether trust fully mediates the effects of trustworthiness and trust propensity on behavioral outcomes. Our meta-analysis of 132 independent samples summarized the relationships between the trust variables and both risk taking and job performance (task performance, citizenship behavior, counterproductive behavior). Meta-analytic structural equation modeling supported a partial mediation model wherein trustworthiness and trust propensity explained incremental variance in the behavioral outcomes when trust was controlled. Further analyses revealed that the trustworthiness dimensions also predicted affective commitment, which had unique relationships with the outcomes when controlling for trust. These results generalized across different types of trust measures (i.e., positive expectations measures, willingness-to-be-vulnerable measures, and direct measures) and different trust referents (i.e., leaders, coworkers).