We report on GCaMP-Rs, a new family of genetically encoded ratiometric calcium indicators that extend the virtues of the GCaMP proteins to ratiometric measurements. We have engineered a tandem construct of calcium-dependent GCaMP and calcium-independent mCherry fluorescent proteins. The tandem design assures that the two proteins localize in the same cellular compartment(s) and facilitates pixelwise ratiometric measurements; however, Förster resonance energy transfer (FRET) between the fluorophores reduces brightness of the sensor by up to half (depending on the GCaMP variant). To eliminate FRET, we introduced a rigid α-helix, the ER/K helix, between GCaMP and mCherry. Avoiding FRET significantly increases the brightness (notably, even at low calcium concentrations), the signal-to-noise ratio, and the dynamic range.

The force-generating mechanism of dynein differs from the forcegenerating mechanisms of other cytoskeletal motors. To examine the structural dynamics of dynein's stepping mechanism in real time, we used polarized total internal reflection fluorescence microscopy with nanometer accuracy localization to track the orientation and position of single motors. By measuring the polarized emission of individual quantum nanorods coupled to the dynein ring, we determined the angular position of the ring and found that it rotates relative to the microtubule (MT) while walking. Surprisingly, the observed rotations were small, averaging only 8.3°, and were only weakly correlated with steps. Measurements at two independent labeling positions on opposite sides of the ring showed similar small rotations. Our results are inconsistent with a classic power-stroke mechanism, and instead support a flexible stalk model in which interhead strain rotates the rings through bending and hinging of the stalk. Mechanical compliances of the stalk and hinge determined based on a 3.3-μs molecular dynamics simulation account for the degree of ring rotation observed experimentally. Together, these observations demonstrate that the stepping mechanism of dynein is fundamentally different from the stepping mechanisms of other well-studied MT motors, because it is characterized by constant small-scale fluctuations of a large but flexible structure fully consistent with the variable stepping pattern observed as dynein moves along the MT.ynein is a molecular motor that walks processively toward the minus end of microtubules (MTs) in an ATP-dependent manner (1-3). Axonemal dyneins drive the motility of eukaryotic cilia and flagella, whereas cytoplasmic dynein is responsible for a wide range of functions within eukaryotic cells, including the retrograde transport of cargo such as autophagosomes in neurons (4), alignment of the mitotic spindle (5, 6), and chromosome segregation during mitosis (7). Disruption of dynein-mediated neuronal transport has been implicated in neurodegeneration (8), and mutations in dynein and dynein-associated proteins can cause a range of diseases, including spinal muscular atrophy (9, 10), lissencephaly (11) and Charcot-Marie-Tooth disease type 2 (reviewed in ref. 12). Despite the importance of dynein function, the mechanism by which dynein walks along the MT is not yet well understood.The motor domains of dynein are formed from six concatenated AAA domains, interrupted by a long, antiparallel, coiled-coil stalk that emerges from AAA4 and terminates in the microtubulebinding domain (MTBD) and the buttress that extends from AAA5 and is thought to stabilize the stalk (13,14). Two motor domains form a dimer via their N-terminal tails (2, 3). ATP binding and hydrolysis drive dynein mechanochemistry; the binding of ATP to AAA1 induces a conformational change leading to dissociation of the MTBD from the MT. Hydrolysis on the dissociated head induces a primed conformation that then rebinds to the MT. The forward step, or power stroke,...

Fluorescence resonance energy transfer (FRET) provides a powerful means to study protein conformational changes. However, the incorporation of an exogenous FRET pair into a protein could lead to undesirable structural perturbations of the native fold. One of the viable strategies to minimizing such perturbations is to use non-natural amino acid based FRET pairs. Previously we have shown that p-cyanophenylalanine (Phe CN ) and tryptophan (Trp) constitute such a FRET pair, useful for monitoring protein folding-unfolding transitions. Herein, we further show that 7-azatryptophan (7AW) and 5-hydroxytryptophan (5HW) can also serve as a FRET acceptor to Phe CN , and the resultant FRET pairs offer certain advantages over Phe CN -Trp. For example, the fluorescence spectrum of 7AW is sufficiently separated from that of Phe CN , making it straightforward to decompose the FRET spectrum into donor and acceptor contributions. Moreover, we show that Phe CN , Trp and 7AW can be used together to form a multi-FRET system, allowing more structural information to be extracted from a single FRET experiment. The applicability of these FRET systems is demonstrated in a series of studies wherein they are employed to monitor the urea-induced unfolding transitions of the villin headpiece subdomain (HP35), a designed ββα motif (BBA5), and the human Pin1 WW domain.

The subdomains of macromolecules often undergo large orientation changes during their catalytic cycles that are essential for their activity. Tracking these rearrangements in real time opens a powerful window into the link between protein structure and functional output. Site-specific labeling of individual molecules with polarized optical probes and measuring their spatial orientation can give insight into the crucial conformational changes, dynamics, and fluctuations of macromolecules. Here we describe the range of single molecule optical technologies that can extract orientation information from these probes, we review the relevant types of probes and labeling techniques, and we highlight the advantages and disadvantages of these technologies for addressing specific inquiries.

Our understanding of molecular motor function has been greatly improved by the development of imaging modalities, which enable real-time observation of their motion at the single-molecule level. Here, we describe the use of a new method, interferometric scattering microscopy, for the investigation of motor protein dynamics by attaching and tracking the motion of metallic nanoparticle labels as small as 20 nm diameter. Using myosin-5, kinesin-1, and dynein as examples, we describe the basic assays, labeling strategies, and principles of data analysis. Our approach is relevant not only for motor protein dynamics but also provides a general tool for single-particle tracking with high spatiotemporal precision, which overcomes the limitations of single-molecule fluorescence methods.