SAT-298 Structural and Functional Analysis of Pheochromocytoma Tumor Suppressor TMEM127 Reveals its Mechanism of Interaction with E3 Ubiquitin Ligases and Substrate MHC-I

Abstract Disclosure: H. Gonzalez Cantu: None. S. Munawar: None. Y. Yuan: None. D. Zhou: None. P.L. Dahia: None. Mutations of TMEM127 confer susceptibility to pheochromocytomas, via a mechanism that is attributed, at least in part, to impaired TMEM127-E3 ubiquitin ligase interactions, leading to reduced degradation of its substrate, the RET oncogene1. MHC-I, a major target of cancer immune surveillance, has emerged as another TMEM127 substrate, in a mechanism congruent with the TMEM127-RET mode of regulation2. As a modulator of cancer associated proteins, understanding how the interactions between TMEM127 and substrates are regulated will shed light on cancer therapeutic strategies, including cancer immunotherapies. Here, we sought to structurally and functionally characterize the TMEM127-E3 ubiquitin ligase-MHC-I interactome using in silico predictions, as well as in vitro and in vivo models. Our in silico structural modeling of the TMEM127 interactome predicted that TMEM127 was an essential component of the complex, bridging the E3 ubiquitin ligase WWP2 with MHC-I. These predictions were validated by in vitro co-immunoprecipitation assays, and further suggested that TMEM127 C-terminal PY motif is involved in these interactions. Flow cytometry analyses of immune and non-immune tissue from whole-body Tmem127 KO mice showed higher cell surface MHC-I levels in Tmem127 KO tissue compared to Tmem127 WT controls, supporting the role of Tmem127 as a component of MHC-I regulation in vivo. Taken together, these data suggest that TMEM127 is a critical structural scaffold molecule of the MHC-I regulatory interactome. We speculate that antagonizing TMEM127-WWP2 interactions may increase the cell surface density of MHC-I and consequently, may serve as a potential therapeutic strategy to augment cancer immune surveillance. References 1.Guo Q, Cheng ZM, Gonzalez-Cantu H, et al. TMEM127 suppresses tumor development by promoting RET ubiquitination, positioning, and degradation. Cell Rep 2023;42(9):113070. DOI: 10.1016/j.celrep.2023.113070.2.Chen X, Lu Q, Zhou H, et al. A membrane-associated MHC-I inhibitory axis for cancer immune evasion. Cell 2023;186(18):3903-3920 e21. DOI: 10.1016/j.cell.2023.07.016. Presentation: Saturday, July 12, 2025