PCLO (piccolo, also known as aczonin) is a large (more than 400kD) multidomain protein of the presynaptic cytomatrix in neurons, that is present in all vertebrate synapses, but is absent from invertebrates.
It contains zinc finger and coiled-coil sequences, as well as N-terminal glutamine-rich sequence, and C-terminal PDZ domain followed by two C2 domains (C2A and C2B).
In vitro binding and transfection experiments suggested that PCLO binds to multiple proteins including profilin and L-type calcium channels.
It is involved in neurotransmitter release and insulin secretion.