FIGURE 6.

Targeting VSFPs to different membrane systems. The plasma membrane (PM) and the inner and outer nuclear membranes (INM and ONM, respectively) are represented by concentric black partial and full circles. For simplicity, the nuclear pores, at which the INM and ONM are fused, are omitted. The ONM is continuous with the endoplasmic reticulum (ER). The perinuclear space (PNS) between the INM and ONM is contiguous with the lumen of the ER. Targeting of VSFPs to the PM is achieved by Ci-VSP (Figure 3). Ideally, targeting signals specific for the INM and ONM can be identified since these two membranes may have distinct and independently regulated potentials. Using different VSFPs in the PM and ONM (or INM) would allow multicolor imaging and simultaneous monitoring of potentials at different membrane systems within a single cell. Shown here is the hypothetical situation of FRET-based VSFP2.3 in the PM (or INM) and intensity-based VSFP3.1TagRFP in the ONM. The dotted arrow and question mark indicate the possibility of detecting synchronous changes in PM and ONM (or INM) potentials using VSFPs.