Co-occurrence of psychiatric disorders is welldocumented. Recent quantitative efforts have moved toward an understanding of this phenomenon, with the 'general psychopathology' or p-factor model emerging as the most prominent characterization. Over the past decade, bifactor model analysis has become increasingly popular as a statistical approach to describe common/shared and unique elements in psychopathology. However, recent work has highlighted potential problems with common approaches to evaluating and interpreting bifactor models. Here, we argue that, when properly applied and interpreted, bifactor models can be useful for answering some important questions in psychology and psychiatry research. We review problems with evaluating bifactor models based on global model fit statistics. We then describe more valid approaches to evaluating bifactor models and highlight three types of research questions for which bifactor models are wellsuited to answer. We also discuss the utility and limits of bifactor applications in genetic and neurobiological research. We close by comparing advantages and disadvantages of bifactor models to other analytic approaches and noting that no statistical model is a panacea to rectify limitations of the research design used to gather data. 1 Sometimes, group factors are called "specific factors." However, "specific factor" more correctly refers to an item's reliable (non-error) variance that is not shared with other items (5).

We have used transient kinetics, nanosecond time-resolved fluorescence resonance energy transfer (FRET), and kinetics simulations to resolve a structural transition in the Dictyostelium myosin II relay helix during the actin-activated power stroke. The relay helix plays a critical role in force generation in myosin, coupling biochemical changes in the ATPase site with the force-transducing rotation of the myosin light-chain domain. Previous research in the absence of actin showed that ATP binding to myosin induces a dynamic equilibrium between a bent prepower stroke state of the relay helix and a straight postpower stroke state, which dominates in the absence of ATP or when ADP is bound. We now ask whether actin binding reverses this transition and if so, how this reversal is coordinated with actin-activated phosphate release. We labeled a Cys-lite Dictyostelium myosin II motor domain with donor and acceptor probes at two engineered Cys residues designed to detect relay helix bending. We then performed transient timeresolved FRET following stopped-flow mixing of actin with labeled myosin, preincubated with ATP. We determined the kinetics of actin-activated phosphate release, using fluorescent phosphatebinding protein. The results show that actin binding to the myosin.ADP.P complex straightens the relay helix before phosphate dissociation. This actin-activated relay helix straightening is reversible, but phosphate irreversibly dissociates from the postpower stroke state, preventing reversal of the power stroke. Thus, relay helix straightening gates phosphate dissociation, whereas phosphate dissociation provides the thermodynamic driving force underlying force production.muscle | time-resolved FRET | structural kinetics T he myosin ATPase family uses the free energy of ATP hydrolysis to power a cycle of changes in myosin structure. These changes perform mechanical work, essential for motility in all eukaryotic cells. The efficient production of work by myosins is controlled by nucleotide and actin binding in coordination with a reversible rotation of the myosin light-chain domain, hypothesized to act as a force transducing "lever arm" (1-3). The myosin relay helix is crucial for this coordination (1, 4, 5).The structure of the relay helix is coupled to nucleotide binding and to the orientation of the light-chain domain. The relay helix is straight when myosin is crystallized with bound nonhydrolyzed ATP, prehydrolysis ATP analogs, or ADP or in the absence of nucleotide (abbreviated apo) and bent when crystallized with bound posthydrolysis ATP analogs (Fig. 1A, nucleotide free, IFMV, and ADP.Vanadate bound, 1VOM, crystal structures shown) (3, 6-9). There is no crystal structure of myosin bound to actin, but the relay helix is straight in a myosin-V crystal structure that is hypothesized to exhibit features of an actin-bound state (10) and also in a 0.8-nm resolution cryo-electron microscopy model of actin-bound myosin (11). In each of these studies, the orientation of the light-chain domain mirrors the structural s...

We describe a nanosecond time-resolved fluorescence spectrometer that acquires fluorescence decay waveforms from each well of a 384-well microplate in 3 min with signal-to-noise exceeding 400 using direct waveform recording. The instrument combines high-energy pulsed laser sources (5-10 kHz repetition rate) with a photomultiplier and high-speed digitizer (1 GHz) to record a fluorescence decay waveform after each pulse. Waveforms acquired from rhodamine or 5-((2-aminoethyl)amino) naphthalene-1-sulfonic acid dyes in a 384-well plate gave lifetime measurements 5- to 25-fold more precise than the simultaneous intensity measurements. Lifetimes as short as 0.04 ns were acquired by interleaving with an effective sample rate of 5 GHz. Lifetime measurements resolved mixtures of single-exponential dyes with better than 1% accuracy. The fluorescence lifetime plate reader enables multiple-well fluorescence lifetime measurements with an acquisition time of 0.5 s per well, suitable for high-throughput fluorescence lifetime screening applications.

Vesicular and tubular transport intermediates regulate organellar cargo dynamics. Transport carrier release involves local and profound membrane remodeling before fission. Pinching the neck of a budding tubule or vesicle requires mechanical forces, likely exerted by the action of molecular motors on the cytoskeleton. Here, we show that myosin VI, together with branched actin filaments, constricts the membrane of tubular carriers that are then released from melanosomes, the pigment containing lysosome-related organelles of melanocytes. By combining superresolution fluorescence microscopy, correlative light and electron microscopy, and biochemical analyses, we find that myosin VI motor activity mediates severing by constricting the neck of the tubule at specific melanosomal subdomains. Pinching of the tubules involves the cooperation of the myosin adaptor optineurin and the activity of actin nucleation machineries, including the WASH and Arp2/3 complexes. The fission and release of these tubules allows for the export of components from melanosomes, such as the SNARE VAMP7, and promotes melanosome maturation and transfer to keratinocytes. Our data reveal a new myosin VI- and actin-dependent membrane fission mechanism required for organelle function.