Many of hardly treatable diseases like Cancer, Alzheimer, AIDS or Diabetes could be back-traced to the failure in the function of specific Proteins. To specifically intervene with such functions we should understand both Structure and the Mechanics of action for "Protein-targets". Investigation of reactions in live-cells or tissues along with identifications of the molecules involved is nowadays one of the primary interest for biologists. There is an urgent request for development of analysis methods which deliver the results efficiently and fast.

FRET (Förster resonance energy transfer) technique provides an optophysiological readout and allows the analysis of macromolecular work or interactions inside of intact living cells. Relatively simple direct detection of the signals and non-invasive character contributes to the explosive growth of FRET popularity.

SMAD proteins are the major signal transducers for the Transforming Growth Factor superfamily of cytokines and their serine/threonine kinase receptors. SMADs mediate the signal from the membrane into the nucleus. Bone Morphogenetic Protein-4 stimulates phosphorylation of SMAD-1, which interacts with Smad4. This complex translocates into the nucleus and regulates transcription of target genes. The new cellular fluorescence biosensors were developed for direct visualization of SMAD-dependent signaling in live mammalian cells. Photometry approach was used to measure small changes in Fluorescence resonance energy transfer between Cyan andYellow fluorescent proteins fused to the Smad1 and Smad4 proteins was used to unravel the temporal aspects of BMP/SMAD signaling. A rate-limiting delay of 2–5 min occurred between BMP activation and SMAD-1 activity. A similar delay was observed in the SMAD-1/SMAD-4 complexation. Further experimentation indicated that the delay is dependent on the MH1 domain and linker of SMAD-1. These results give new insights into the dynamics of the BMP receptor –SMAD-1/-4 signaling process and provide a new tool for studying SMADs.