Mechanisms of USP7/MAGEL2 Complex Assembly and Its Mutational Disruption in Neurodevelopmental Diseases

The WASH complex regulates endosomal trafficking and is linked to several neurodevelopmental diseases, including Prader-Willi syndrome, Schaaf-Yang syndrome, and Hao-Fountain syndrome. Its function is tightly controlled by ubiquitination, maintained by the multi-subunit MUST complex containing both a ubiquitin ligase (MAGEL2/TRIM27) and a deubiquitinase (USP7). However, the mechanism underlying the MUST complex assembly remains poorly understood. In this study, we investigate the assembly of USP7 and MAGEL2 components of the MUST complex using NMR spectroscopy, isothermal titration calorimetry, X-ray crystallography, and cellular assays. We show that the USP7/MAGEL2 interaction is bipartite and multivalent. Two distinct domains of USP7, TRAF and UBL1-2, recognize two unstructured but evolutionarily conserved regions of MAGEL2, one of which contains multiple TRAF-binding sites. Furthermore, we determine the high-resolution crystal structure of the TRAF/MAGEL2 complex and identify Hao-Fountain syndrome-linked mutations in USP7 that disrupt USP7/MAGEL2 complex formation in vitro and in cells. These findings provide mechanistic insight into the pathogenic basis of Hao-Fountain syndrome and related Schaaf-Yang and Prader-Willi syndromes.