Guanine nucleotide exchange factor (GEF) for Rho and Rac. GEF proteins activate small GTPases by exchanging bound GDP for free GTP. Controls actin polymerization via the two heteromeric complexes WAVE and actin-related protein (ARP) 2/3 (PubMed:18308939). Involved in cytoskeletal reorganization required for cell shape (e.g. trichome and cotyledon) control and tissue development (PubMed:11826302). Promotes polarized growth and cell-cell adhesion in the leaf epidermis probably by promoting the formation of endoplasmic reticulum (ER) exit site (ERES) and/or trafficking between the ER and Golgi (PubMed:21109438). Triggers ARAC3/ROP6 activation required for auxin-mediated inhibition of PIN2 internalization during gravitropic responses (, PubMed:22683260). Homodimer. Component of SCAR/WAVE and ARP2/3 complexes. Interacts directly with ARAC4/ROP2, ARAC1/ROP3, ARAC5/ROP4, ARAC6/ROP5, ARAC8/ROP10, ARAC9/ROP8, SCAR1, SCAR2, SCAR3, SCAR4, ABI1, ABI2, ABI3 and ABI4 (PubMed:17267444, PubMed:18308939). Binds to the inactive GDP-bound form of ARAC3/ROP6 (PubMed:22683260). Accumulates at, and promotes the formation of, a specialized domain of the endoplasmic reticulum termed the ER exit site (ERES). Expressed ubiquitously, in roots and aerial organs. The DOCKER domain may mediate some GEF activity. Dwarf in spk1-1 and spk1-5 (PubMed:22683260). Seedling lethal with trichome, cotyledon, and leaf-shape defects due to a misregulation of laterally clustered foci of microtubules and polarized growth in epidermal cells. Sporophytic sterility. Trichomes show a reduced branch number and an irregular swelling along the stalk and branches (PubMed:11826302, PubMed:18308939). Enlarged endoplasmic reticulum (ER) cisternae and constitutively activated ER stress response (PubMed:21109438). Increased lateral root density and retarded gravitropic responses associated with PIN2 internalization. In spk1-4, enhanced cell shape changes induced by ROP6 overexpression, and reduced active ROP6 (GTP-bound) levels (PubMed:22683260). Belongs to the DOCK family.