Cy3-Tris NTA compound is a sensitive fluorescent chelator for detecting polyhistidine-labeled proteins in cells, solution, and solid surfaces with the addition of certain metal ions (such as Ni2+, Co2+ etc.) as a mediator. It is the chelator of our HIS Lite™ Cy3 Tris NTA-Ni Complex (#12620). Fluorescent tris-NTA compounds provide an efficient method for site-specific and stable noncovalent fluorescence labeling of polyhistidine-tagged proteins. In contrast to the transient binding of conventional mono-NTA, the multivalent interaction of tris-NTA conjugated fluorophores form a much more stable complex with polyhistidine-tagged proteins. The high selectivity of tris-NTA compounds toward cumulated histidines enables the selective labeling of proteins in cell lysates and on the surface of live cells. Fluorescent tris-NTA conjugates can be applied for the analysis of a ternary protein complex in solution and on surfaces. In combination with other color tris-NTA compounds (such as #12655 and #12657), it can be used for multicolor analysis of polyhistidine-tagged proteins. The transition metal ions (e.g., Ni2+)-mediated complexation of polyhistidine-labeled proteins with fluorescent tris-NTA conjugates provides a sensitive reporter for detecting and monitoring protein-protein interactions in real time.