Protein-protein interactions (PPIs) are important in close to all vital processes in the cell. Ranging from stable protein complexes to transient interactions the studies of PPIs is not only critical in the understanding of protein functions but also difficult to monitor. Bimolecular fluorescence complementation (BiFC) allows PPIs to be studied by analyzing fluorescence as a result of an interaction. Non fluorescent fragments of YFP is fused to the protein targets and upon assembly into a complex fluorescence is emitted.

We show that the BiFC method in S. cerevisiae can be used to confirm the tetramerization of human aquaporin0 (hAQP0) which was used as a positive control for the assay. Also, no tetramerization is observed between hAQP0 and hAQP2, acting as a negative control.

The method was then applied to the the well-known interaction between aquaporin 0 and calmodulin (CaM). Whereas previous studies have an in vitro approach, we can show the complex formation of hAQP0-CaM in vivo. By truncating the C-terminal of hAQP0 just before the putative CaM interaction site the fluorescence is drastically decreased as expected when the interaction site is removed.

S. cerevisiae is an excellent system for PPI studies and in the case of hAQP0-CaM it has been shown to also work for interactions between a membrane protein and a soluble protein. This particular complex can now be studied more easily compared to the mammalian cell systems used previously and the consequence of minor changes in either protein can be quantitated and related to differences in the affinity of the complex.