Long wavelength engineered fluorescent proteins

A method for screening the effects of a test compound on ion channel activity in a cell comprising the steps of,

• i) providing a cell comprising an ion channel of interest and an engineered green fluorescent protein comprising an amino acid sequence at least 85% identical to the amino acid sequence of the Aequorea green fluorescent protein set forth in SEQ ID NO:2 and which differs from SEQ ID NO:2 by
• (1) at least one first substitution at position T203, wherein the substituting amino acid is selected from the group consisting of H, Y, W and F,
• (2) at least one second substitution at position H148, wherein the substituting amino acid is selected from the group consisting of R, G, Q, A, N, and K, and
• (3) at least one third amino acid substitution at position V150, wherein the substituting amino acid is selected from the group consisting of A, C, M, G, L, Q, S, T and N;
• and
• ii) contacting said cell with the test compound,
• iii) measuring fluorescence from said engineered green fluorescent protein, and
• iv) comparing the fluorescence of said engineered green fluorescent protein in said cell to the fluorescence of a control engineered green fluorescent protein introduced into a control cell wherein the control cell is not contacted with the test compound.