UAS regulatory sequences drive expression of a cytosolic topology-sensor protein that comprises YFP(2) (a non-fluorescent split fluorescent protein fragment) fused to the leucine zipper dimerization domain of the Saccharomyces cerevisiae GCN4 gene (ZIP). If a complementary sensor protein containing a split fluorescent protein fragment that is complementary to YFP(2) (such as YFP(1)) and a ZIP domain is expressed in the same cell on the same side of the membrane (in the cytoplasm), the two ZIP domains promote association of the two split fluorescent protein fragments and a functional fluorescent protein may be reconstituted.