FAM105A/OTULINL is a pseudodebuiquitinase of the OTU-class that localizes to the ER membrane

Pseudoenzymes have been identified across a diverse range of enzyme classes and fulfill important cellular functions. Examples of pseudoenzymes exist within ubiquitin conjugating and deubiquitinase protein families. Here we characterize FAM105A/OTULINL, the only putative pseudodeubiquitinase (DUB) of the ovarian tumour protease (OTU domain) family in humans. The crystal structure of FAM105A revealed that the OTU domain possesses structural deficiencies in both active site and substrate-binding infrastructure predicted to impair normal DUB function. We confirmed the absence of catalytic function against all ubiquitin linkages and an inability of FAM105A to bind ubiquitin compared to catalytically active FAM105B/OTULIN. FAM105A co-localized with KDEL markers and Lamin B1 at the endoplasmic reticulum (ER) and nuclear envelope, respectively. Accordingly, the FAM105A interactome exhibited significant enrichment in proteins localized to the ER/outer nuclear, Golgi and vesicular membranes. In light of undetectable deubiquitinase activity, we posit that FAM105A/OTULINL functions through its ability to mediate protein-protein interactions. FAM105A is an OTU-class pseudo-deubiquitinase with a disrupted catalytic triad and undetectable cleavage activity for any diubiquitin linkage. Surface conservation predicts that FAM105A has evolved an adaptor function unrelated to a direct interaction with ubiquitin.