Catalytic subunit of the heterodimeric RalGAP1 complex which acts as a GTPase activator for the Ras-like small GTPases RALA and RALB. Component of the heterodimeric RalGAP1 complex with RALGAPB. Heterodimerization is required for activity. Interacts with the HLH region of TCF3/isoform E12 (By similarity). Translocated to the nucleus, when associated with TCF3/E12. Widely expressed. The disease is caused by variants affecting the gene represented in this entry.