We introduce a method for optically imaging intracellular proteins at nanometer spatial resolution. Numerous sparse subsets of photoactivatable fluorescent protein molecules were activated, localized (to È2 to 25 nanometers), and then bleached. The aggregate position information from all subsets was then assembled into a superresolution image. We used this method-termed photoactivated localization microscopy-to image specific target proteins in thin sections of lysosomes and mitochondria; in fixed whole cells, we imaged vinculin at focal adhesions, actin within a lamellipodium, and the distribution of the retroviral protein Gag at the plasma membrane.

Chimeric antigen receptors (CARs) targeting CD19 have mediated dramatic anti-tumor responses in hematologic malignancies, but tumor regression has rarely occurred using CARs targeting other antigens. It remains unknown whether the impressive effects of CD19 CARs relate to greater susceptibility of hematologic malignancies to CAR therapies, or superior functionality of the CD19 CAR itself. We discovered that tonic CAR CD3ζ phosphorylation, triggered by antigen-independent clustering of CAR scFvs, can induce early exhaustion of CAR T cells that limits anti-tumor efficacy. Such activation is present to varying degrees in all CARs studied, with the exception of the highly effective CD19 CAR. We further identify that CD28 costimulation augments, while 4-1BB costimulation ameliorates, exhaustion induced by persistent CAR signaling. Our results provide biological explanations for the dramatic anti-tumor effects of CD19 CARs and for the observations that CD19.BBz CAR T cells are more persistent than CD19.28z CAR T cells in clinical trials.

We report a photoactivatable variant of the Aequorea victoria green fluorescent protein (GFP) that, after intense irradiation with 413-nanometer light, increases fluorescence 100 times when excited by 488-nanometer light and remains stable for days under aerobic conditions. These characteristics offer a new tool for exploring intracellular protein dynamics by tracking photoactivated molecules that are the only visible GFPs in the cell. Here, we use the photoactivatable GFP both as a free protein to measure protein diffusion across the nuclear envelope and as a chimera with a lysosomal membrane protein to demonstrate rapid interlysosomal membrane exchange.

We combined photoactivated localization microscopy (PALM) with live-cell single-particle tracking to create a new method termed sptPALM. We created spatially resolved maps of single-molecule motions by imaging the membrane proteins Gag and VSVG, and obtained several orders of magnitude more trajectories per cell than traditional single-particle tracking enables. By probing distinct subsets of molecules, sptPALM can provide insight into the origins of spatial and temporal heterogeneities in membranes.

We have investigated properties relevant to quantitative imaging in living cells of five green fluorescent protein (GFP) variants that have been used extensively or are potentially useful. We measured the extinction coefficients, quantum yields, pH effects, photobleaching effects, and temperature-dependent chromophore formation of wtGFP, alphaGFP (F99S/M153T/V163A), S65T, EGFP (F64L/S65T), and a blue-shifted variant, EBFP (F64L/S65T/Y66H/Y145F). Absorbance and fluorescence spectroscopy showed little difference between the extinction coefficients and quantum yields of wtGFP and alphaGFP. In contrast, S65T and EGFP extinction coefficients made them both approximately 6-fold brighter than wtGFP when excited at 488 nm, and EBFP absorbed more strongly than the wtGFP when excited in the near-UV wavelength region, although it had a much lower quantum efficiency. When excited at 488 nm, the GFPs were all more resistant to photobleaching than fluorescein. However, the wtGFP and alphaGFP photobleaching patterns showed initial increases in fluorescence emission caused by photoconversion of the protein chromophore. The wtGFP fluorescence decreased more quickly when excited at 395 nm than 488 nm, but it was still more photostable than the EBFP when excited at this wavelength. The wtGFP and alphaGFP were quite stable over a broad pH range, but fluorescence of the other variants decreased rapidly below pH 7. When expressed in bacteria, chromophore formation in wtGFP and S65T was found to be less efficient at 37 degrees C than at 28 degrees C, but the other three variants showed little differences between 37 degrees C and 28 degrees C. In conclusion, no single GFP variant is ideal for every application, but each one offers advantages and disadvantages for quantitative imaging in living cells.

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The ability to visualize, track, and quantify molecules and events in living cells with high spatial and temporal resolution is essential for understanding biological systems. Only recently has it become feasible to carry out these tasks due to the advent of fluorescent protein technology. Here, we trace the development of highly visible and minimally perturbing fluorescent proteins that, together with updated fluorescent imaging techniques, are providing unprecedented insights into the movement of proteins and their interactions with cellular components in living cells.

The reliance of modern microscopy techniques on photoactivatable fluorescent proteins prompted development of mCherry variants that are initially dark but become red fluorescent after violetlight irradiation. Using ensemble and single-molecule characteristics as selection criteria, we developed PAmCherry1 with excitation/emission maxima at 564/595 nm. Compared to other monomeric red photoactivatable proteins, it has faster maturation, better pH stability, faster photoactivation, higher photoactivation contrast and better photostability. Lack of green fluorescence and single-molecule behavior make monomeric PAmCherry1 a preferred tag for twocolor diffraction-limited photoactivation imaging and for super-resolution techniques such as oneand two-color photoactivated localization microscopy (PALM). We performed PALM imaging using PAmCherry1-tagged transferrin receptor expressed alone or with photoactivatable GFPtagged clathrin light chain. Pair correlation and cluster analyses of the resulting PALM images identified ≤200 nm clusters of transferrin receptor and clathrin light chain at ≤25 nm resolution and confirmed the utility of PAmCherry1 as an intracellular probe.Genetically encoded `photoactivatable' fluorescent proteins (PAFPs) make up a small category of fluorescent proteins 1 , but are beginning to find uses far and above those of normal' fluorescent proteins 2 . With initially little or no fluorescence within their associated spectral detection window, photoactivatable proteins can be switched on by irradiation with violet light. Thus they are useful for spatially pulse-labeling subpopulations of molecules in cells in complement to photobleaching applications and can provide other useful features such as a high contrast over background in the photoactivated region and circumvention of fluorescence contributions from newly synthesized, nonactivated PAFPs. PAFPs and photoswitchable dyes also provide probes necessary for high-resolution optical techniques, such as photoactivated localization microscopy (PALM) 3 , fluorescence photoactivated localization microscopy (FPALM) 4 , stochastic reconstruction microscopy (STORM) 5 , PALM with independent running acquisition (PALMIRA) 6 RESULTS Development of photoactivatable mCherry variantsWe analyzed data on color transitions of red fluorescent proteins to the respective nonfluorescent chromoproteins that had been generated by mutagenesis 1 and identified the corresponding crucial amino acid positions on the basis of the mCherry structure 18 . Positions spatially close to the chromophore, such as 148, 165, 167 and 203 (numbering is in accordance with GFP alignment; Supplementary Fig. 1 online), appear to be major molecular determinants of color 10,19 . We hypothesized that mutagenesis of mCherry at these positions might convert it to a photoactivatable red probe and performed saturating mutagenesis at these positions using the overlap extension approach.We screened the resulting bacterial library of the site-specific mCherry mutants by fluorescence-activated ...