Green fluorescent protein (GFP)-based fluorescent indicators for Ca 2+ offer significant promise for monitoring Ca 2+ in previously unexplored organisms, tissues, and submicroscopic environments because they are genetically encoded, function without cofactors, can be targeted to any intracellular location, and are bright enough for single-cell imaging. These probes use simple GFP variants, circularly permuted GFP (cpGFP), in which the amino and carboxyl portions have been interchanged and reconnected by short spacers between the original termini, or pairs of GFP variants that permit fluorescence resonance energy transfer (FRET). Yellow cameleons (YCs) use FRET between cyanand yellow-emitting variants of Aequorea GFP (cyan fluorescent protein [CFP] and yellow fluorescent protein [YFP], respectively). YCs are composed of a linear combination of CFP, calmodulin (CaM), a glycylglycine linker, the CaM-binding peptide of myosin light-chain kinase (M13), and YFP. Binding of Ca 2+ to the CaM moiety of the YC initiates an intramolecular interaction between the CaM and the M13 domains, causing the chimeric protein to shift from an extended conformation to a more compact one, thereby increasing the efficiency of FRET from CFP to YFP. This technique is amenable to emission ratioing, which is more quantitative than single-wavelength monitoring. YC3.60 was engineered to enhance its performance as a fluorescent Ca 2+ indicator and here we describe the use of this cameleon to image rapid changes in intracellular free Ca 2+ concentration ([Ca 2+ ] i ) within HeLa cells. FRET imaging is performed using a laser-scanning confocal microscope.
MATERIALSIt is essential that you consult the appropriate Material Safety Data Sheets and your institution's Environmental Health and Safety Office for proper handling of equipment and hazardous material used in this protocol.
ReagentsCa 2+ -free medium For fast and simultaneous acquisition of cpVenus173 and CFP images from HeLa cells expressing YC3.60, this camera, which comprises three charge-coupled device (CCD) chips (RGB: red, green, and blue) and a prism, can be used. The cpVenus173 and CFP images are captured by the G and B chips, respectively. In addition, to improve spatial resolution along the z-axis, a spinning-disk unit (e.g., CSU21 from Yokogawa Electric Corporation) can be placed in front of the camera.Diode-pumped solid-state laser (430 nm) (Melles Griot) Glass-bottomed dishes (35 mm) (Matsunami Glass Ind., Ltd., D111300) Image acquisition software to control the Ashura 3CCD camera (e.g., . These include intensity of the laser, scanning speed, binning, and sensitivity of the camera.In our experience, the intensity of the laser should be attenuated greatly using neutral-density filters for Ca 2+ imaging on a timescale of subseconds. Photobleaching does not happen with a laser that is sufficiently attenuated.6. Observe fluorescence signals from CFP and cp173Venus by the blue and green channels of the 3CCD camera, respectively. 7. Set 4 × 4 binning for high-sensitive fluorescence ...