Ypq1 and Ssh4 transmembrane interaction is regulated by the PQ motif. (A) Architecture of prokaryotic SemiSWEET and eukaryotic SWEETs, which are the most well-studied members of the PQ-loop protein family. SemiSWEET protomers and SWEET THBs are colored in pink and green. (B) Cartoon showing key helices in SemiSWEET transitioning from the outward-open to occluded to inward-open conformations. TM1 and TM3 in the second protomer of the SemiSWEET transporter unit corresponds to TM5 and TM7, respectively, in Ypq1. Major helical movements are influenced by the PQ motif. (C) Left: The effect of PQ motif mutation on Ypq1-GFP complementation pairs. Right: Quantification (±SD, n = 3). OE, overexpression. PQmut, P229S,Q230R. (D) The effect of PQ motif mutation on Ypq1-GFP subcellular localization. Scale bar, 2 µm. (E) Predicted structure of Ypq1 modeled on three conformations of SemiSWEET. Membrane interfaces are shown in red and blue, as predicted by the Positioning of Membranes in Proteins server (https://opm.phar.umich.edu/ppm_server).