Catalytic subunit of the heterodimeric RalGAP1 complex which acts as a GTPase activator for the Ras-like small GTPases RALA and RALB (By similarity). May interact with the HLH region of TCF3/isoform E12. Component of the heterodimeric RalGAP1 complex with RALGAPB. Heterodimerization is required for activity (By similarity). Interacts with the HLH region of TCF3/isoform E12. Translocated to the nucleus, when associated with TCF3/E12. Expressed during embryogenesis. Expressed in the adult brain, particularly in neurons of the cortex and hippocampus. Expression decreased during development, but persists in the adult brain. May be a partial isoform 1. Was initially thought to act as a transcriptional regulator via its interaction with TCF3/E12. Truncated C-terminus.