Charge complementation pairs support a transmembrane interaction between Ypq1 and Ssh4. (A) Conceptual model of charge complementation. (B) Flow cytometry–based quantification of Ypq1-GFP degradation within complementation pairs. EV, empty vector; OE, overexpression. (C) Top: Degradation of Ypq1-GFP within complementation pairs. Bottom: Quantification (±SD, n = 3). Vec, empty vector. (D) Subcellular localization of Ypq1-GFP coexpressed with Ssh4 mutants after 6 h Lys starvation. Scale bar, 2 µm. (E) Cell counts from D. No degradation (VM, vacuole membrane); complete degradation (vacuole lumen, VL); partial degradation (partial, VL + VM). n > 300 for each strain. (F) A model of the binding interface between Ypq1 TM5 and Ssh4 TM.