Component of the C9orf72-SMCR8 complex, a complex that has guanine nucleotide exchange factor (GEF) activity and regulates autophagy. In the complex, C9orf72 and SMCR8 probably constitute the catalytic subunits that promote the exchange of GDP to GTP, converting inactive GDP-bound RAB8A and RAB39B into their active GTP-bound form, thereby promoting autophagosome maturation. The C9orf72-SMCR8 complex also acts as a regulator of autophagy initiation by interacting with the ULK1/ATG1 kinase complex and modulating its protein kinase activity. As part of the C9orf72-SMCR8 complex, stimulates RAB8A and RAB11A GTPase activity in vitro (By similarity). Positively regulates initiation of autophagy by regulating the RAB1A-dependent trafficking of the ULK1/ATG1 kinase complex to the phagophore which leads to autophagosome formation. Acts as a regulator of mTORC1 signaling by promoting phosphorylation of mTORC1 substrates. Plays a role in endosomal trafficking. May be involved in regulating the maturation of phagosomes to lysosomes. Promotes the lysosomal localization and lysosome-mediated degradation of CARM1 which leads to inhibition of starvation-induced lipid metabolism (By similarity). Regulates actin dynamics in motor neurons by inhibiting the GTP-binding activity of ARF6, leading to ARF6 inactivation. This reduces the activity of the LIMK1 and LIMK2 kinases which are responsible for phosphorylation and inactivation of CFL1/cofilin, leading to cofilin activation. Positively regulates axon extension and axon growth cone size in spinal motor neurons. Required for SMCR8 protein expression and localization at pre- and post-synaptic compartments in the forebrain, also regulates protein abundance of RAB3A and GRIA1/GLUR1 in post-synaptic compartments in the forebrain and hippocampus (By similarity). Plays a role within the hematopoietic system in restricting inflammation and the development of autoimmunity. Component of the C9orf72-SMCR8 complex, at least composed of C9orf72, SMCR8 and WDR41 (By similarity). The complex is formed of two protomers, each individually consisting of one molecule each of C9orf72, SMCR8 and WDR41 (By similarity). The protomers homodimerize via an interaction between C9orf72 (via C-terminus) and SMCR8 (via N-terminus) (By similarity). Within each protomer SMCR8 (via DENN domain) acts as a bridging protein between WDR41 (via C-terminus and N-terminus) and C9orf72 (via C-terminus) (By similarity). The C9orf72-SMCR8 complex associates with the ULK1/ATG1 kinase complex. Interacts with ULK1/ATG1 kinase complex members ULK1, ATG13 and RB1CC1. Interacts with SMCR8; the interaction is direct (By similarity). Interacts with HNRNPA1, HNRNPA2B1 and UBQLN2. Interacts with small Rab GTPase RAB1A; the interaction mediates recruitment of RAB1A to the ULK1/ATG1 kinase complex. Also interacts with small Rab GTPase RAB7A. Interacts with cofilin (By similarity). Interacts with GTP-binding proteins ARF1 and ARF6. Interacts with the DLG4/PSD-95 (By similarity). Interacts with CARM1 (via PH domain-like fold) (By similarity). Detected in the cytoplasm of neurons from brain tissue. Detected in the nucleus in fibroblasts. Associates with cytoplasmic stress granules following cellular stress. During corticogenesis, transitions from being predominantly cytoplasmic to a more even nucleocytoplasmic distribution.