Catalytic subunit of the heterodimeric RalGAP2 complex which acts as a GTPase activator for the Ras-like small GTPases RALA and RALB. Component of the heterodimeric RalGAP2 complex with RALGAPB. Heterodimerization is required for activity. Highly expressed in lung, liver, testis and thymus with lower levels in brain and heart (at protein level).