Fig. 2. Fluorescence responses of GCS sensor proteins. (a) Fluorescence responses of 1 μM GCS-1 <5 min after the addition of 0, 0.4, 0.8, 1.6, and 3.2 μM Cu²⁺. (b) Fluorescence responses of 0.5 μM protein <5 min after the addition of 0, 1, 10, and 50 μM Cu²⁺. cpOPT: circularly permuted green fluorescent protein lacking the ATCUN motif; wtOPT: wild-type green fluorescent protein. (c) Fluorescence responses of 0.5 μM protein 1 h after the addition of 0, 1, 10, and 50 μM Cu²⁺. (d) Fluorescence responses of 0.1 μM protein 1 h after the addition of 0 and 0.5 μM Cu²⁺. The error bars correspond to the standard error of the mean of three independent measurements.