Stimulates the GTPase activity of RAP2A. Promotes reorganization of the actin cytoskeleton and recruits DLG4 to F-actin. Contributes to the regulation of dendritic spine morphogenesis. Interacts (via PDZ domain) with EPHA4 (via PDZ motif); controls neuronal morphology through regulation of the RAP1 (RAP1A or RAP1B) and RAP2 (RAP2A, RAP2B or RAP2C) GTPases (By similarity). Interacts with DLG4, PDLIM5, PDLIM7 and LZTS3. Interacts with the actin cytoskeleton. Associated with the actin cytoskeleton. Detected at synapses and dendritic spines of cultured hippocampal neurons. Detected in brain (at protein level). Ubiquitinated and degraded by the SCF(BTRC) following phosphorylation by PLK2. Phosphorylated at Ser-1367 by CDK5, creating a docking site for the POLO box domains of PLK2. Subsequently, PLK2 binds and phosphorylates SIPA1L1, leading to ubiquitination and degradation by the proteasome.