Investigating interactions mediated by the presynaptic protein bassoon in living cells by Foerster's resonance energy transfer and fluorescence lifetime imaging microscopy.

Neuronal synapses are highly specialized structures for communication between nerve cells. Knowledge about their molecular organization and dynamics is still incomplete. The large multidomain protein Bassoon plays a major role in scaffolding and organizing the cytomatrix at the active zone of neurotransmitter release in presynaptic boutons. Utilizing immunofluorescence techniques, we show that Bassoon is essential for corecruitment of its synaptic interaction partners, C-terminal binding protein 1/brefeldin A-dependent ADP-ribosylation substrate and CAZ-associated structural protein, into protein complexes upon heterologous expression in COS-7 cells. A combination of Foerster's resonance energy transfer and fluorescence lifetime imaging microscopy in the time domain was adopted to investigate the potential for the association of these proteins in the same complexes. A direct physical association between Bassoon and CtBP1 could also be observed at synapses of living hippocampal neurons. Simultaneous analysis of fluorescence decays of the donor and the acceptor probes along with their decay-associated spectra allowed a clear discrimination of energy transfer.