May modulate calcium-mediated postsynaptic signals. Complex formation with APBA2 and APP, stabilizes APP metabolism and enhances APBA2-mediated suppression of beta-APP40 secretion, due to the retardation of intracellular APP maturation. Directly interacts with APBA2. Forms a tripartite complex with APBA2 and APP (By similarity). Interacts with low affinity with KLC1. Most prominent in the postsynaptic specializations of asymmetric (type I) synapses with both axodendritic and axospinous localization. Restricted to the brain. In the cerebral cortex, found in the somas and neuropil of all layers. Expressed at highest levels in neurons of cortical layer 5 and, at lower levels, in neurons of the upper layers. Highly expressed in Purkinje cells. Also found in a few scattered interneurons throughout the granule cell layer and occasionally in neurons in the molecular layer (at protein level). In all layers, high levels in a subpopulation of presumptive GABAergic neurons (based on morphology). Binds synaptic Ca(2+) with its cytoplasmic domain.