Reduces the ATPase activity of VCP. Necessary for the fragmentation of Golgi stacks during mitosis and for VCP-mediated reassembly of Golgi stacks after mitosis. May play a role in VCP-mediated formation of transitional endoplasmic reticulum (tER). Inhibits the activity of CTSL (in vitro). Together with UBXN2B/p37, regulates the centrosomal levels of kinase AURKA/Aurora A levels during mitotic progression by promoting AURKA removal from centrosomes in prophase. Also, regulates spindle orientation during mitosis. Part of a ternary complex containing STX5A, NSFL1C and VCP. NSFL1C forms a homotrimer that binds to one end of a VCP homohexamer. The complex binds to membranes enriched in phosphatidylethanolamine-containing lipids and promotes Golgi membrane fusion. Interaction with VCIP135 leads to dissociation of the complex via ATP hydrolysis by VCP. Binds ubiquitin and mono-ubiquitinated proteins via its N-terminal UBA-like domain when bound to VCP (By similarity). Predominantly nuclear in interphase cells. Bound to the axial elements of sex chromosomes in pachytene spermatocytes. A small proportion of the protein is cytoplasmic, associated with Golgi stacks. Localizes to centrosome during mitotic prophase and metaphase. Phosphorylated during mitosis. Phosphorylation inhibits interaction with Golgi membranes and is required for the fragmentation of the Golgi stacks during mitosis (By similarity). Belongs to the NSFL1C family.