Location of calcium transporters at presynaptic terminals.

The plasma membrane ATP-driven Ca2+ pump (PMCA) and the Na+/Ca2+ exchanger (NCX) are the major means of Ca2+ extrusion at presynaptic nerve terminals, but little is know about the location of these transporters relative to the major sites of Ca2+ influx, the transmitter release sites. We used immunocytochemistry to identify these transport proteins in a calyx-type presynaptic nerve terminal from the ciliary ganglion of the chick. The PMCA clusters were localized to the transmitter release sites, as identified by staining for the secretory vesicle-specific protein synaptotagmin I. This colocalization was not due to the presence of the pump on the secretory vesicle itself because membrane fractionation of chick brain synaptosomes demonstrated comigration of the pump with surface membrane and not vesicle markers. In contrast, the NCX did not colocalize with synaptotagmin but tended to be located at nonsynaptic regions of the terminal. The PMCA location, near the transmitter release sites, suggests that it plays a role in priming the release site by maintaining a low free Ca2+ level, facilitating the dissociation of the ion from its binding sites. The PMCA may also replenish external Ca2+ in the synaptic cleft following periods of synaptic activity. In contrast, the NCX location suggests a role in the rapid emptying of cytoplasmic Ca2+ uptake organelles which serve as the main line of defence against high free Ca2+.