Synaptic vesicle (SV) clusters are essential for neurotransmitter release at high rates of stimulation [1] with a role in pathological states, such as epilepsy. It was proposed that the resting SV cluster is organized as a liquid phase maintained by interactions of the intrinsically disordered region (IDR) of synapsin. To address how the SV liquid phase is regulated during synaptic activity we used intersectin (ITSN) as model protein. Intersectin, which is accumulated in the SV cluster, interacts with the synapsin IDR through a conserved phospho-regulated switch mechanism. In dephospho-intersectin, synapsin-binding to an ITSN SH3 domain is autoinhibited while in phospho-intersectin it strongly binds synapsin.