Npl4 is part of the heterodimer UN along with Ufd1 which is involved in the recruitment of p97, an AAA ATPase, for tasks involving the ubiquitin pathway. Npl4 has a ubiquitin-like domain which has within its structure a beta-grasp fold with a helical insert [1].

1: Detailed structural insights into the p97-Npl4-Ufd1 interface. Isaacson RL, Pye VE, Simpson P, Meyer HH, Zhang X, Freemont PS, Matthews S; J Biol Chem. 2007;282:21361-21369. PMID:17491009