Rho GTPases are small G protein members of the Ras superfamily that regulate cytoskeletal organization, cell-cycle progression, and gene expression, while their dysregulation drives tumor formation and metastasis. The P-Rex (PI[3,4,5]P3-dependent Rac exchanger) proteins are large multi-domain guanine nucleotide exchange factors (GEFs) that are key activators of the Rho GTPase Rac1, controlling cell migration. There are two P-Rex homologues, P-Rex1 and P-Rex2, whose dysregulation has been identified as drivers of metastasis in multiple cancers. P-Rex1 is frequently overexpressed in melanoma, breast, and prostate cancers, while P-Rex2 is the third most mutated protein in primary melanomas and is also highly mutated in breast, pancreatic, and lung cancers. Inferring how these mutations in the P-Rex proteins drive cancer progression is currently hampered by our incomplete understanding of how the P-Rex proteins' activity is regulated.
Despite sharing identical domain arrangement and approximately 60% sequence identity, the P-Rex proteins are differentially regulated. P-Rex1 is maintained in an autoinhibited conformation, which has recently been solved by Cryo-EM, while P-Rex2 forms a co-inhibitory complex with the tumor suppressor protein PTEN, although the structure of this co-inhibitory complex remains elusive. Both P-Rex proteins are synergistically activated at the membrane by PI[3,4,5]P3, downstream of receptor tyrosine kinases (RTK), and dimeric GꞵƔ, downstream of G-protein-coupled receptors (GPCR). The necessity of both PI[3,4,5]P3 and GꞵƔ for P-Rex protein activation is unique among GEFs and enables the integration of signals downstream of RTKs and GPCRs. However, the structural basis by which these second messengers relieve P-Rex protein inhibition and activate their GEF activity remains poorly understood.
In this poster, I will present data generated in the first year of my PhD that begins to uncover the structural basis of P-Rex protein regulation. First, I will focus on my work to understand the structure of the P-Rex2 PTEN co-inhibitory complex. Secondly, I will present data working towards uncovering the structure of the active P-Rex1 Rac1 GꞵƔ complex.