Ubiquitin-binding protein involved in protein ubiquitination, sorting and degradation (PubMed:2111732, PubMed:8890162, PubMed:19805280, PubMed:18508771, PubMed:20508643, PubMed:27044889). Acts as a ubiquitinated substrate-recruiting adapter for chaperone ATPase CDC48 by binding mono- or polyubiquitin chains (PubMed:15096053, PubMed:16427015, PubMed:16428438, PubMed:27044889). Depending on the context, promotes or prevents proteasomal degradation of ubiquitinated proteins (PubMed:2111732, PubMed:8890162, PubMed:19805280, PubMed:27044889). Involved in the ubiquitin fusion degradation (UFD) pathway by promoting the degradation of ubiquitinated proteins (PubMed:2111732, PubMed:8890162, PubMed:19805280, PubMed:27044889). Involved in the mitochondria-associated degradation pathway (MAD) by promoting the degradation of several ubiquitinated membrane proteins (PubMed:27044889). By competing with UFD2 to bind CDC48, prevents the multi-ubiquitination and subsequent degradation of UFD2-dependent substrates (PubMed:16427015). Required for ribophagy, a process which relocalizes ribosomal particles into the vacuole for degradation in response to starvation (PubMed:20508643). Involved in the ubiquitin-mediated sorting of membrane proteins into multivesicular bodies (MVBs) (PubMed:18508771). In addition, plays an essential role in maintaining cellular ubiquitin levels (PubMed:7615550, PubMed:16427015, PubMed:16428438, PubMed:18508771, PubMed:19805280). May affect indirectly the degradation of ubiquitinylated proteins by regulating cellular ubiquitin levels (PubMed:7615550, PubMed:23525333). Forms a complex composed of CDC48, NPL4, UFD1, DOA1, SHP1 and deubiquitinase OTU1; within the complex interacts with CDC48 (PubMed:16427015). Interacts (via PUL domain) with CDC48 (via C-terminus); the interaction is direct (PubMed:8890162, PubMed:16427015, PubMed:16428438, PubMed:27044889, PubMed:19805280, PubMed:20508643). Forms a complex composed of CDC48, DOA1, deubiquitinase UBP3 and probably BRE5; within the complex interacts with CDC48 and UBP3 (PubMed:20508643). May form a complex composed of VPS27, HSE1 and DOA1 (PubMed:18508771). Interacts with HSE1 (via SH3 domain) (PubMed:18508771). Interacts (via WD repeats and PFU domain) with ubiquitin; the interaction is direct (PubMed:15096053, PubMed:16427015, PubMed:16428438, PubMed:21070969). Interacts with ubiquitinated FZO1 but not unmodified FZO1; the interaction recruits FZO1 to CDC48 and promotes FZO1 proteasomal degradation (PubMed:27044889). Predominantly localizes to the cytoplasm. Probably localizes to endosomes and mitochondria in a transient manner. The WD repeats mediate interaction with ubiquitin. The PUL domain is composed of 6 armadillo-like repeats and mediates the interaction with CDC48 C-terminus. The PFU domain mediates interaction with ubiquitin. Severely reduces cell growth in response to misfolded protein, translation inhibition-induced, heat or mitochondrial oxidative stresses but not in response to ER stress (PubMed:16427015, PubMed:18508771, PubMed:16428438, PubMed:27044889). Present with 6800 molecules/cell in log phase SD medium. Belongs to the WD repeat PLAP family.