1.A.55 The Synaptic Vesicle-Associated Ca2+ Channel, Flower (Flower) Family
Synaptic vesicle (SV) exo- and endocytosis are tightly coupled to sustain neurotransmission in presynaptic terminals, and both are regulated by Ca2+. Ca2+ influx triggered by voltage-gated Ca2+ channels is necessary for SV fusion. However, extracellular Ca2+ has also been shown to be required for endocytosis. The intracellular Ca2+ levels (<1 μM) that trigger endocytosis are typically much lower than those (>10 μM) needed to induce exocytosis, and endocytosis is inhibited when the Ca2+ level exceeds 1 μM. Yao et al., (2009) identified and characterized a 4 TMS transmembrane protein named 'Flower', associated with SVs that, upon SV fusion, localizes at periactive zones. Loss of Flower results in impaired intracellular resting Ca2+ levels and impaired endocytosis. Flower multimerizes and is able to form a channel to control Ca2+ influx. Yao et al. (2009) proposed that Flower functions as a Ca2+ channel to regulate synaptic endocytosis and hence couples exo- with endocytosis. Homologues may be present in various intrarcellular membranes. The Flower family is also called the Cg6252-P Family.