Targeting Rab27A with covalent inhibitors and Exocytosis TArgeting Chimeras (ExoTACs) for lung cancer treatment

Small monomeric guanosine triphosphatases (GTPases) are a group of hydrolase enzymes that bind to and hydrolyse a guanosine triphosphate (GTP) to form guanosine diphosphate (GDP). GTP-bound small GTPases can recruit specific effectors through protein-protein interactions (PPIs) to trigger signalling cascades in cells. Rab27A is a small GTPase protein playing a key role in secretion of exosomes, membrane-bound vesicles responsible for intercellular communication and cell migration. GTP-bound Rab27A mediates the release of exosomes by binding to its effectors such as Slp2a. Several studies have shown that loss of Rab27A expression impairs cancer cell invasion and metastasis due to reduction of pro-invasive exosomes. Therefore, targeting Rab27A is a promising strategy to overcome metastasis and drug resistance in lung cancer. However, Rab27A is a very challenging target due to its conformational flexibility and the lack of canonical binding pockets beyond the widely conserved GTP binding site. The host lab (Tate) used a covalent ligand approach to target two non-conserved cysteines in proximity to the effector binding site on Rab27 isoforms and identified the first bona fide Rab27A ligand series, with optimized hits exhibiting high selectivity for Cys123, strong interference with the Rab27A-Slp2 effector PPI, and moderate proteome-level selectivity (unpublished results). In this project, I will (1) optimize these starting points to design the first proteome-wide selective Rab27A inhibitor to interfere with its binding to effectors, subsequently (2) characterizing its impact on exosome secretion, invasion and growth of lung cancer cells. In the second part of the project, I will (3) develop a new class of bifunctional molecules termed Exocytosis TArgeting Chimeras (ExoTACs), linking a selective Rab27A ligand with a ligand to a target protein, enabling for the first time compound-induced exocytosis of a target protein.