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ut ca w icINTRODUCTION investigated. Moreover, it is not known to which degree and by which mechanism Ca2+ influx is affected at the level of individual synapses and how this might contribute to the exocytic deficit.Sensory encoding in the auditory and visual system of verte-brates relies on transformation of graded receptor potentials Several mechanisms may explain the impairment of fast exocy-tosis in IHCs ofBsnDEx4/5mutants. First, mutant AZsmay containfusion of synaptic vesicles. It is not well understood how Ca2+ channel clustering and synaptic vesicle docking are organized. Here, we studied structure and function of hair cell ribbon synapses following genetic disruption of the presynaptic scaffold protein Bassoon. Mutant synapses—mostly lacking the ribbon—showed a reduction in membrane-proximal vesicles, with ribbonless synapses affected more than ribbon-occupied synapses. Ca2+ channels were also fewer at mutant synapses and appeared in abnormally shaped clusters. Ribbon absence reduced Ca2+ channel numbers at mutant and wild-type synapses. Fast and sustained exocytosis was reduced, notwithstanding normal coupling of the remaining Ca2+ channels to exocytosis. In vitro recordings revealed a slight impairment of vesicle replenishment. Mechanistic modeling of the in vivo data independently supported morphological and functional in vitro findings. We conclude that Bassoon and the ribbon (1) create a large number of release sites by organizing Ca2+ channels and vesicles, and (2) promote vesicle replenishment. et al., 2008; Nouvian et al., 2006; Sterling and Matthews, 2005). Aside from its major component, RIBEYE (Khimic