Acts as a GEF (guanine nucleotide exchange factor) for the Rho family of small GTP-binding proteins (G proteins) that stimulates the dissociation of GDP to enable subsequent binding of GTP (PubMed:11948427, PubMed:1549351, PubMed:20709748, PubMed:28630045, PubMed:30190425). Additionally, appears to chaperone the processing and/or trafficking of small GTPases containing a C-terminal polybasic region independently of GEF activity (PubMed:20709748, PubMed:21242305). Targets include RAP1A/RAP1B, RHOA, RHOB, RHOC, RAC1 and KRAS (PubMed:11948427, PubMed:1549351, PubMed:20709748, PubMed:24415755). Regulates mitochondrial dynamics by controlling RHOT function to promote mitochondrial fission during high calcium conditions (PubMed:27716788). Able to promote the Ca(2+) release from the endoplasmic reticulum via both inositol trisphosphate (Ins3P) and ryanodine sensitive receptors leading to a enhanced mitochondrial Ca(2+) uptake (PubMed:24349085). {ECO:0000269|PubMed:11948427, ECO:0000269|PubMed:1549351, ECO:0000269|PubMed:20709748, ECO:0000269|PubMed:21242305, ECO:0000269|PubMed:24349085, ECO:0000269|PubMed:24415755, ECO:0000269|PubMed:27716788, ECO:0000269|PubMed:28630045, ECO:0000269|PubMed:30190425, ECO:0000305|PubMed:30190425}. [Isoform 1]: Acts as a GEF (guanine nucleotide exchange factor) for unprenylated RHOA (PubMed:24415755, PubMed:28630045, PubMed:30190425). Chaperones the entry and passage of small GTPases through the prenylation pathway (PubMed:20709748). Recognizes the last amino acid in the GTPase C-terminal CAAX motif with a preference for 'Leu' over 'Met', indicating involvement in the geranylgeranylation pathway (PubMed:24415755). {ECO:0000269|PubMed:20709748, ECO:0000269|PubMed:24415755, ECO:0000269|PubMed:28630045, ECO:0000269|PubMed:30190425}. [Isoform 2]: Acts as a GEF (guanine nucleotide exchange factor) for prenylated RHOA (PubMed:21242305, PubMed:28630045, PubMed:30190425). Acts as a GEF for RHOC (PubMed:21242305). Chaperones the downstream trafficking and/or processing of small newly prenylated GTPases (PubMed:20709748). Escorts RAC1 to the nucleus (PubMed:12551911). {ECO:0000269|PubMed:12551911, ECO:0000269|PubMed:20709748, ECO:0000269|PubMed:21242305, ECO:0000269|PubMed:28630045, ECO:0000269|PubMed:30190425}. This is the function of Ensembl gene identifier ENSG00000138698 (RAP1GDS1, Rap1 GTPase-GDP dissociation stimulator 1).