Analysis of protein complex hierarchy in living cells

The identification and analysis of protein complexes is usually achieved by performing in vitro experiments. We describe a translocation-based method for studying protein complexes in living cells. Annexin A4, a phospholipid- and calcium-binding protein, translocates from the cytoplasm and nucleoplasm to plasma and nuclear membranes in response to elevated intracellular calcium levels. By fusing a bait protein, for instance, a core component of a protein complex or a similarly relevant peptide, to the annexin, translocation of both bait and its interacting target proteins are readily monitored in living cells in response to a single treatment. Proteins of interest are fused to a variety of fluorescent proteins suitable for multiparameter imaging. Using this generally applicable approach, we were able to visualize the formation of protein complexes in their natural environment. Specifically, we detected the hierarchical assembly of four-component protein complexes in single cells.