3D localized photoactivation of pa-GFP in living cells using two-photon interactions.

We report about two-photon activation of a photoactivatable derivative of the Aequorea Victoria green fluorescent protein (paGFP). This special form of the molecule increases its fluorescence intensity when excited by 488 nm after irradiation with high intensity light at 413 nm. The aim in this work was to evaluate the use of two-photon interactions for confining the molecular switching of pa-GFP in the bright state. Therefore experiments were performed using fixed and living cells which were expressing the paGFP fluorophore and microspheres whose surface was modified by specific adsorption of the chromophores. The molecular switches were activated in a range of wavelength from 720 nm to 840 nm. The optimal wavelength for activation was then chosen for cell imaging. A comparison between the conventional activation and two-photon mode demonstrates clearly the better three- dimensional (3D) confinement and the possibility of selection of cell volumes of interest. This enables molecular trafficking studies at high signal to noise ratio.