Functions as a guanine nucleotide exchange factor (GEF) that promotes the exchange of GDP to GTP, converting inactive GDP-bound small GTPases into their active GTP-bound form (PubMed:12628187, PubMed:16464467). Involved in regulation of adherens junction between cells (PubMed:12628187). Plays a role in cell migration (PubMed:20679435). Has a higher guanine nucleotide exchange factor activity compared to other isoforms. Interacts with nucleotide-free Rap1; functions as a guanine nucleotide exchange factor (GEF) for Rap1 (PubMed:12628187). Interacts (via DOCKER domain) with RAC1; functions as a guanine nucleotide exchange factor (GEF) for RAC1 (PubMed:16464467). Interacts with the SH3 domain of CRK (PubMed:12628187). Interacts with FASLG (PubMed:19807924). Interacts with ELMO2 and EPHA2; mediates activation of RAC1 by EPHA2 (PubMed:20679435). Interacts with USH1C (via PDZ 1 domain) (PubMed:16464467). Colocalizes with EPHA2, RHOG and CTTN/cortactin at the tip of protrusions in migrating cells. Widely expressed at low level. Highly expressed in skeletal muscle, prostate and ovary. May be specifically expressed in the brain and eye. The DOCKER domain mediates interaction with small GTPases like RAC1 and is required for their activation. Belongs to the DOCK family. Truncated N-terminus.