Figure 1A. Schematic representation of all BiFC combinations studied in this work (lime green cartoons: the two most commonly used BiFC pairs β'-COPVN•Dsl3pVC and Dsl1pVN•ε-COPVC; dashed lines: all other BiFC pairs presented in this work). All BiFC-tagged proteins are listed in the table. B. Bimolecular fluorescence complementation between subunits of the Dsl complex and the COPI coat. The BiFC pair β'-COPVN•Dsl3pVC yielded fluorescent foci that showed dramatically reduced fluorescence intensity in the COPI binding-deficient dsl1-5xWA mutant. Schematic yeast cell representations depict the typical BiFC foci localization patterns. Scale bar 10 μm. C. Quantification of the β'-COPVN•Dsl3pVC BiFC signals in wild type cells and in COPI binding-deficient dsl1-5xWA and dsl1-Δlasso mutants (as shown exemplarily in (B)). Mean values + SEM of at least three independent experiments (n=3-8) are displayed. Statistical analysis was carried out by comparing the number of cells with BiFC signals over integrated thresholds to those without signals (*=p<0,05). dsl1 mutant strains show a significant decrease in brightness, spot number and spot size in comparison with their corresponding DSL1 wild type strains (two-sample t-test). The dsl1-5xWA mutation also led to comparable signal reduction in cells carrying the VNI152L variant in the β'-COPVN I152L•Dsl3pVC combination. D. Subcellular localization of different BiFC-tagged protein pairs. The Dsl1pVN•Dsl3pVC combination involving subunits of the ER-resident Dsl1 complex exhibited typical ER localization. Sec16pVN•Sec16pVC signals in a diploid heterozygous strain displayed an pattern typical for ER exit sites, while diploid cells carrying the α-COPVN•α-COPVC BiFC pair showed a Golgi-like fluorescence pattern. Scale bar 10 μm. E. The localization of COPI•Dsl BiFC foci compared to the autophagy marker Ape1p. Double fluorescence micrographs of the BiFC pair β'-COPVN•Dsl3pVC and the autophagy marker mRFPApe1p are superimposed with the DIC image. BiFC-YFP signals are pseudocolored green. Fluorescent signals appeared distinct from each other. Also no co-localization was observed when we analyzed COPI•COPII(Sec16p) BiFC spots in cells expressing mRFP tagged APE1. Scale bar 10 µm. F. Growth effects of Venus fragment tags in dsl1 mutant cells. Serial tenfold dilutions of liquid cell cultures were spotted on agar plates and incubated at 30 °C for two days. The images in the second row show that β'COPVN producing cells expressing the dsl1-5xWA mutation could not grow. The complementation of β'COPVN by its cognate interaction partners Dsl1pVC and Dsl3pVC, but not by non-cognate BiFC partners (Sec24pVC, Sec16pVC or VCRer1p), suppressed these growth defects. Spot assays of exemplary BiFC partners are depicted. Asterisk: Dsl1pVC carrying the dsl1-5xWA defect was expressed from a plasmid. See Fig EV1, EV2 and Appendix Fig S1 for full data display. G. BiFC signal quantifications of the strains presented in (F). β'-COPVN yielded BiFC fluorescence signals with all tested complementation partners. This rules out the possibility that the lack of suppression observed in F simply reflects the inability of the BiFC partners to form a complex, and shows that the successful complementation of the split-YFP fragments does not per se suppress the synthetic lethal effect of the VN tag in dsl1-5xWA cells. Mean values + SEM of at least three independent experiments (n=3-8) are displayed. H. BiFC signal intensities greatly depend on which BiFC combination is analyzed. Fluorescence intensities of COPI•Dsl BiFC pairs (left part of H) were much higher than those of COPII•Dsl BiFC pairs (right part of H). I. Localization of Dsl1pCFP in β'-COPVN•Dsl3pVCcells. An overall ER localization of Dsl1pCFP was retained in β'-COPVN•Dsl3pVCcells, while some of the signal was found in the foci. BiFC-YFP signals are pseudocolored red. Scale bar 10 µm.