A fluorescent fusion polypeptide, wherein said fluorescent fusion polypeptide is capable of changing its localization within a cell from cytoplasmic membrane to a retention vesicle upon an increase in the concentration of a second messenger in the cytoplasm of the cell, and comprises, from N- to C-terminus, a membrane localization peptide, a fluorescent peptide, a second messenger transduction protein binding peptide, and a reticulum retention signal peptide;
- wherein the second messenger is diacylglycerol;
- wherein the membrane localization peptide is a human IL-2R comprising the amino acid sequence of SEQ ID NO:
- 17;
- and
- wherein the second messenger transduction protein binding peptide binds to PKCδ and comprises the amino acid sequence of SEQ ID NO:
- 19.