TBC1D22B Regulates ER‐to‐Golgi Trafficking via RAB1B Inactivation and Promotes Oncogenic Programs in Breast Cancer

TBC1D22B is a GTPase-activating protein (GAP) associated with poor prognosis in breast cancer (BC). Using complementary proximity-labeling and co-immunoprecipitation proteomics, the TBC1D22B interactome in BC cells is defined, revealing strong enrichment in components of the ER-to-Golgi trafficking machinery, endosomal transport, and adhesion-related pathways. Functional assays, using the Retention Using Selective Hooks (RUSH) system, demonstrate that TBC1D22B inhibits ER-to-Golgi transport in a GAP-dependent manner. Mechanistic studies identify RAB1B as a direct target of TBC1D22B, and RAB1B silencing phenocopies the trafficking defects caused by TBC1D22B overexpression. In 3D culture, TBC1D22B promotes spheroid growth in a manner dependent on its GAP activity and not replicated by its paralog TBC1D22A. Transcriptomic profiling reveals that TBC1D22B overexpression triggers repression of a core module of extracellular matrix and adhesion-related genes, consistent with altered secretory activity. Importantly, this transcriptional program is also evident in primary Luminal BC with high TBC1D22B expression, highlighting a conserved and functionally relevant signature. Together, these findings establish TBC1D22B as a regulator of ER-to-Golgi trafficking via RAB1B and implicate it in oncogenic transcriptional remodeling and tumor growth.