Methods to Study the Unique SOCS Box Domain of the Rab40 Small GTPase Subfamily

Despite the critical role of Rab GTPases for intracellular transport, the vast majority of proteins within this family remain poorly characterized, including the Rab40 subfamily. Often recognized as atypical Rabs, the Rab40 family of proteins are unlike any other small GTPase because they contain a C-terminal suppressor of cytokine signaling (SOCS) box. It is well established that this SOCS domain in other proteins mediates an interaction with the scaffold protein Cullin5 in order to form a E3 ubiquitin ligase complex critical for protein ubiquitylation and turnover. Although the function of SOCS/Cullin5 complexes has been well defined in several of these other proteins, this is not yet the case for the Rab40 family of proteins. We have previously shown that the Rab40b family member plays an important role during three-dimensional (3D) breast cancer cell migration. To further this knowledge, we began to investigate the SOCS-dependent role of Rab40b during cell migration. Here, we describe an unbiased approach to identify potential Rab40b/Cullin5 substrates. We anticipate that this method will be useful for studying the function of other Rab40 family members as well as other SOCS box containing proteins.