Roger Heim

Fluorescent indicators for Ca2+ based on green fluorescent proteins and calmodulin.

Important Ca2+ signals in the cytosol and organelles are often extremely localized and hard to measure. To overcome this problem we have constructed new fluorescent indicators for Ca2+ that are genetically encoded without cofactors and are targetable to specific intracellular locations. We have dubbed these fluorescent indicators ‘cameleons’. They consist of tandem fusions of a blue- or cyan-emitting mutant of the green fluorescent protein (GFP),, calmodulin, the calmodulin-binding peptide M13 (ref. 6), and an enhanced green- or yellow-emitting GFP. Binding of Ca2+ makes calmodulin wrap around the M13 domain, increasing the fluorescence resonance energy transfer (FRET) between the flanking GFPs. Calmodulin mutations can tune the Ca2+ affinities to measure free Ca2+ concentrations in the range 10−8 to 10−2 M. We have visualized free Ca2+ dynamics in the cytosol, nucleus and endoplasmic reticulum of single HeLa cells transfected with complementary DNAs encoding chimaeras bearing appropriate localization signals. Ca2+ concentration in the endoplasmic reticulum of individual cells ranged from 60 to 400 µM at rest, and 1 to 50 µM after Ca2+ mobilization. FRET is also an indicator of the reversible intermolecular association of cyan-GFP-labelled calmodulin with yellow-GFP-labelled M13. Thus FRET between GFP mutants can monitor localized Ca2+ signals and protein heterodimerization in individual live cells.

Understanding, improving and using green fluorescent proteins

Green fluorescent proteins (GFPs) are presently attracting tremendous interest as the first general method to create strong visible fluorescence by purely molecular biological means. So far, they have been used as reporters of gene expression, tracers of cell lineage, and as fusion tags to monitor protein localization within living cells. However, the GFP originally cloned from the jellyfish Aequorea victoria has several nonoptimal properties including low brightness, a significant delay between protein synthesis and fluorescence development, and complex photoisomerization. Fortunately, the protein can be re-engineered by mutagenesis to ameliorate these deficiencies and shift the excitation and emission wavelengths, creating different colors and new applications.

Engineering green fluorescent protein for improved brightness, longer wavelengths and fluorescence resonance energy transfer

BACKGROUND Variants of the green fluorescent protein (GFP) with different colors would be very useful for simultaneous comparisons of multiple protein fates, developmental lineages and gene expression levels. The simplest way to shift the emission color of GFP is to substitute histidine or tryptophan for the tyrosine in the chromophore, but such blue-shifted point mutants are only dimly fluorescent. The longest wavelengths previously reported for the excitation and emission peaks of GFP mutants are 488 and 511 nm, respectively. RESULTS Additional substitutions, mainly in residues 145-163, have improved the brightness of the blue-shifted GFP mutants with histidine and tryptophan in place of tyrosine 66. Separate mutations have pushed the excitation and emission peaks of the most red-shifted mutant to 504 and 514 nm, respectively. At least three different colors of GFP mutants can now be cleanly distinguished from each other under the microscope, using appropriate filter sets. A fusion protein consisting of linked blue- and green-fluorescent proteins exhibits fluorescence resonance energy transfer, which is disrupted by proteolytic cleavage of the linker between the two domains. CONCLUSIONS Our results demonstrate that the production of more and better GFP variants is possible and worthwhile. The production of such variants facilitates multicolor imaging of differential gene expression, protein localization or cell fate. Fusions between mutants of different colors may be useful substrates for the continuous in situ assay of proteases. Demonstration of energy transfer between GFP variants is an important step towards a general method for monitoring the mutual association of fusion proteins.

UNDERSTANDING STRUCTURE-FUNCTION RELATIONSHIPS IN THE AEQUOREA VICTORIA GREEN FLUORESCENT PROTEIN

Publisher Summary Learning the physiological role of green fluorescent protein (GFP) and its interaction with aequorin could help understand how to create mutants that exhibit efficiency energy transfer and how to effectively control dimerization. The interaction of GFP with aequorin is readily reversible and is stabilized by high protein and salt concentrations—conditions likely to be encountered within the light-emitting organelles of Aequorea victoria. Both GFP and aequorin can dimerize under appropriate conditions, and it is the dimerized forms that are believed to interact. The molecular details of the interaction of aequorin with GFP are unknown, although it has been suggested that a C-terminal hydrophobic patch, deriving from amino acids 206,221, and 223, or a stretch of negative electrostatic potential could be plausible interaction domains. Although the significance of the heterogeneity of both GFP and aequorin has largely been ignored, it is possible that it favors the correct association of GFP with aequorin. At least one site of heterogeneity in the Aequorea-derived GFP nucleotide residues occurs at a position involved in GFP Dimerization. Isoform variation at this position was split between positively and negatively charged amino acids, which would favor the selective association of different isoforms.