Figure 2.

Rate models of the voltage-sensitive fluorescent proteins VSFP2.3 and VSFP3.1. (A) Four-state model (Model I): S_ON and S_OFF are the voltage-dependent ON and OFF rate coefficients of the voltage sensor, R_ON and R_OFF, the ON and OFF rate coefficients of the fluorescence reporter. (B) Six-state model (Model II): the voltage sensing domain undergoes fast (rate coefficients: S_1ON, S_1OFF) and slow (rate coefficients: S_2ON, S_2OFF) ON/OFF transitions. (R_ON, R_1OFF, R_2ON, R_OFF) are voltage-independent rate coefficients for transitions of the fluorescence reporter. (C) Eight-state model (Model III): the voltage sensing domain moves between four states, the deactivated state (S−), the primary activated state (S+), the secondary activated state (S++), and the secondary deactivated state (S−−) (see text for details).