The HRD4 gene was identical to NPL4, a gene previously implicated in nuclear transport. Using a diverse set of substrates and direct ubiquitination assays, analysis revealed that HRD4/NPL4 is required for a poorly characterised step in ER-associated degradation after ubiquitination of target proteins but before their recognition by the 26S proteasome [1]. Npl4p physically associates with Cdc48p via Ufd1p to form a Cdc48p-Ufd1p-Npl4p complex. The Cdc48-Ufd1-Npl4 complex functions in the recognition of several polyubiquitin-tagged proteins and facilitates their presentation to the 26S proteasome for processive degradation or even more specific processing.

1: HRD4/NPL4 is required for the proteasomal processing of ubiquitinated ER proteins. Bays NW, Wilhovsky SK, Goradia A, Hodgkiss-Harlow K, Hampton RY; Mol Biol Cell 2001;12:4114-4128. PMID:11739805

2: Cloning and characterization of the gene encoding human NPL4, a protein interacting with the ubiquitin fusion-degradation protein (UFD1L). Botta A, Tandoi C, Fini G, Calabrese G, Dallapiccola B, Novelli G; Gene 2001;275:39-46. PMID:11574150