Guanine nucleotide exchange factor (GEF) for RAP1A, RAP1B and RAP2A small GTPases that is activated by binding cAMP. Seems not to activate RAB3A. Involved in cAMP-dependent, PKA-independent exocytosis through interaction with RIMS2 (By similarity). Interacts with RIMS1 and RIMS2. Probably part of a complex with RIMS2 and GTP-activated RAB3A (By similarity). Predominantly expressed in brain and adrenal gland. Isoform 2 is expressed in liver. Isoform 1 is expressed in liver at very low levels. The N-terminal nucleotide phosphate binding region cAMP 1 has a much lower affinity for cAMP as compared to cAMP 2. The DEP domain is involved in membrane localization independent from regulation by cAMP. Produced by alternative promoter usage. Promoter analysis was carried out in mouse. Produced by alternative promoter usage. Promoter analysis was carried out in mouse. Produced by alternative splicing of isoform 1.