Plasma membrane-associated small GTPase which cycles between active GTP-bound and inactive GDP-bound states (PubMed:24352656). In its active state, binds to a variety of effector proteins to regulate cellular responses such as secretory processes, phagocytosis of apoptotic cells, epithelial cell polarization, neurons adhesion, migration and differentiation, and growth-factor induced formation of membrane ruffles. Rac1 p21/rho GDI heterodimer is the active component of the cytosolic factor sigma 1, which is involved in stimulation of the NADPH oxidase activity in macrophages. Essential for the SPATA13-mediated regulation of cell migration and adhesion assembly and disassembly. Stimulates PKN2 kinase activity. In concert with RAB7A, plays a role in regulating the formation of RBs (ruffled borders) in osteoclasts. In glioma cells, promotes cell migration and invasion. Required for atypical chemokine receptor ACKR2-induced LIMK1-PAK1-dependent phosphorylation of cofilin (CFL1) and for up-regulation of ACKR2 from endosomal compartment to cell membrane, increasing its efficiency in chemokine uptake and degradation. In podocytes, promotes nuclear shuttling of NR3C2; this modulation is required for a proper kidney functioning. In neurons, is involved in dendritic spine formation and synaptic plasticity (PubMed:24352656, PubMed:26969129). In hippocampal neurons, involved in spine morphogenesis and synapse formation, through local activation at synapses by guanine nucleotide exchange factors (GEFs), such as ARHGEF6/ARHGEF7/PIX (PubMed:12695502). In synapses, seems to mediate the regulation of F-actin cluster formation performed by SHANK3. In neurons, plays a crucial role in regulating GABA(A) receptor synaptic stability and hence GABAergic inhibitory synaptic transmission through its role in PAK1 activation and eventually F-actin stabilization (By similarity). GTP + H2O = GDP + H(+) + phosphate Regulated by guanine nucleotide exchange factors (GEFs) which promote the exchange of bound GDP for free GTP, GTPase activating proteins (GAPs) which increase the GTP hydrolysis activity, and GDP dissociation inhibitors which inhibit the dissociation of the nucleotide from the GTPase. GTP hydrolysis is stimulated by ARHGAP30 and ARHGAP44. Interacts with the GEF proteins PREX1, FARP1, FARP2, DOCK1, DOCK2 and DOCK7, which promote the exchange between GDP and GTP, and therefore activate it. Part of a quaternary complex containing PARD3, some PARD6 protein (PARD6A, PARD6B or PARD6G) and some atypical PKC protein (PRKCI or PRKCZ), which plays a central role in epithelial cell polarization. Found in a trimeric complex composed of DOCK1 and ELMO1, which plays a central role in phagocytosis of apoptotic cells. Interacts with RALBP1 via its effector domain. Interacts with BAIAP2, BAIAP2L1, PLXNB1, CYFIP1/SRA-1 and DEF6. Probably found in a ternary complex composed of DSCAM, PAK1 and RAC1. Interacts with DSCAM; the interaction requires PAK1. Interacts with TBC1D2. Interacts with UNKL. Interacts with USP6. Interacts with SPATA13. Interacts with ITGB4. Interacts with the GTP-bound form of RAB7A. Interacts with ARHGEF2. Interacts with ARHGEF16; mediates activation of RAC1 by EPHA2. Interacts with NOXA1. Interacts with S100A8 and calprotectin (S100A8/9). Interacts with ARHGDIA; the interaction is induced by SEMA5A, mediated through PLXNB3 and inactivates and stabilizes RAC1. Interacts with PACSIN2. Interacts with ITGB1BP1 (By similarity). Interacts with the GEF protein RASGRF2, which promotes the exchange between GDP and GTP, and therefore activates it. Interacts with PARD6A, PARD6B and PARD6G in a GTP-dependent manner. Part of a complex with MAP2K3, MAP3K3 and CCM2. Interacts with NISCH. Interacts with PIP5K1A. Interacts (GTP-bound form preferentially) with PKN2 (via the REM repeats); the interaction stimulates autophosphorylation and phosphorylation of PKN2. Interacts with SRGAP2. Interacts with PLXNB3. Interacts (when active) with PPP5C (via TPR repeats); activates PPP5C phosphatase activity and translocates PPP5C to the cell membrane. Interacts with RAPH1 (via Ras associating and PH domains) (By similarity). Interacts with MTSS2 (via IMD domain); this interaction may be important to potentiate PDGF-induced RAC1 activation. Interacts (GTP-bound form) with SH3RF3. Interacts with PAK2 (By similarity). Interacts (GTP-bound form) with SH3RF1 (PubMed:22959435). Found in a complex with SH3RF1, MAPK8IP1/JIP1, MAP3K11/MLK3, MAP2K7/MKK7 and MAPK8/JNK1 (PubMed:23963642). Interacts (both active GTP- or inactive GDP-bound forms) with SH3RF2 (By similarity). Interacts (GTP-bound form preferentially) with CYRIB (By similarity). Interacts with DOCK4 (via DOCKER domain); functions as a guanine nucleotide exchange factor (GEF) for RAC1 (By similarity). Interacts with GARRE1 (By similarity). Interacts with RAP1GDS1 (By similarity). Inner surface of plasma membrane possibly with attachment requiring prenylation of the C-terminal cysteine (By similarity). Found in the ruffled border (a late endosomal-like compartment in the plasma membrane) of bone-resorbing osteoclasts (By similarity). Localizes to the lamellipodium in a SH3RF1-dependent manner. In macrophages, cytoplasmic location increases upon CSF1 stimulation (PubMed:17116687). Activation by GTP-binding promotes nuclear localization (By similarity). Widely expressed. Expressed in the neocortical neurons in the developing brain. The effector region mediates interaction with DEF6. GTP-bound active form is ubiquitinated by HACE1, leading to its degradation by the proteasome. Conditional knockout of Rac1 in the telencephalic ventricular zone of embryos leads to primary microcephaly. Self-renewal, survival, and differentiation of telencephalic neural progenitor cells is affected. Belongs to the small GTPase superfamily. Rho family.