In association with ufd-1 and ATPase cdc-48.1 and/or cdc-48.2, involved in the cytoplasmic elimination of misfolded proteins exported from the ER (PubMed:16647269, PubMed:22768338). This pathway, known as ERAD, prevents the activation of the unfolded protein response (UPR) caused by the accumulation of misfolded proteins in the ER (PubMed:16647269, PubMed:22768338). During S phase and in association with ufd-1, cdc-48.1 and/or cdc-48.2 and ubxn-3, ensures the degradation of DNA licensing factor cdt-1 after the initiation of DNA replication and thus the disassembly of the DNA replication CGM helicase complex by promoting the dissociation from chromatin of several of its components including cdc-45 and sld-5 (PubMed:18728180, PubMed:21981920, PubMed:26842564, PubMed:28368371). Regulates ubxn-3 nuclear localization during S phase (PubMed:26842564). Forms a complex composed of ubxn-3, ufd-1, npl-4.2 and cdc-48.1; within the complex, interacts with ufd-1 and ubxn-3 (By similarity). Interacts with ufd-1 (PubMed:16647269). Interacts with elc-1/elongin C; the interaction may mediate the interaction between the npl-4-ufd-1-cdc-48 complex and the E3 ubiquitin ligase cul-2 complex (By similarity). Localizes to the cytoplasm during mitosis. Nuclear localization upon nuclear membrane re-assembly is cdc-48-dependent. Simultaneous RNAi-mediated knockdown of npl-4.1 and npl-4.2 causes embryonic lethality (PubMed:16647269, PubMed:26842564). In embryos, DNA replication is partially impaired causing a delay in S phase progression in P0, AB and P1 cells; simultaneous RNAi-mediated knockdown of DNA replication checkpoint kinases chk-1 or atl-1 suppresses the delay in S phase (PubMed:18728180, PubMed:26842564). During S phase, prevents DNA replication licensing factor cdt-1 down-regulation and causes cdt-1 accumulation on mitotic chromosomes (PubMed:21981920). Impairs dissociation from the chromatin of components of the DNA replication machinery, including cdc-45, GINS complex component sld-5 and CMG helicase component mcm-3, resulting in their persistent association with chromatin throughout embryonic mitosis (PubMed:21981920, PubMed:26842564, PubMed:28368371). Abnormal ubxn-3 localization into punctate structures in the nucleus (PubMed:26842564). Reduces ufd-1 expression in embryos (PubMed:21981920, PubMed:26842564). Simultaneous RNAi-mediated knockdown of npl-4.1 and npl-4.2 in adults causes a proliferation arrest of mitotic germline cells in the gonad with formation of rad-51 foci on chromatin (PubMed:18728180). Induces the unfolded protein response and increases sensitivity to tunicamycin-induced ER stress (PubMed:16647269). Causes accumulation of misfolded protein cpl-1 in the ER (PubMed:22768338). Belongs to the NPL4 family. There is another gene, npl-4.1, which has a 99% identical sequence making it difficult to identify the specific function for each protein.