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. Additionally, appears to chaperone the processing and/or trafficking of small GTPases containing a C-terminal polybasic region independently of GEF activity. Targets include RAP1A/RAP1B, RHOA, RHOB, RHOC, RAC1 and KRAS. Regulates mitochondrial dynamics by controlling RHOT function to promote mitochondrial fission during high calcium conditions. 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. Acts as a GEF (guanine nucleotide exchange factor) for unprenylated RHOA. Chaperones the entry and passage of small GTPases through the prenylation pathway. 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. May also recognize prenylated GTPases. Acts as a GEF (guanine nucleotide exchange factor) for prenylated RHOA. Acts as a GEF for RHOC. Chaperones the downstream trafficking and/or processing of small newly prenylated GTPases. Escorts RAC1 to the nucleus. Interacts with RABL3. Interacts with RHOT1. Interacts with unprenylated RHOA; the interaction is direct. Interacts with RAP1A. Interacts with KRAS. Interacts with RAC1. Interacts with RAP1B. Preferentially interacts with unprenylated GTPases that will become geranylgeranylated. May also interact with prenylated GTPases. Interacts with prenylated RHOA; the interaction is direct and in a 1:1 stoichiometry. Interacts with RAP1A. Interacts with KRAS. Interacts with RAC1. Interacts with RAP1B. Preferentially interacts with prenylated GTPases. Nuclear import is dependent on complexing with a GTPase containing a C-terminal polybasic region. Brain. The N-terminus is blocked. Forms covalent cross-links mediated by transglutaminase TGM2, between a glutamine and the epsilon-amino group of a lysine residue, forming homopolymers and heteropolymers.