How does the Scar/WAVE complex control actin protrusions and cell migration? A combined cell biology and cryo-EM approach.

We seek to understand cell migration by crawling, a process which is important throughout growth, life and death. Early in life, embryos depend on migration for their shape and structure, and new nerve connections form as the ends of neurons crawl towards one another. Normal life is maintained by cell migration - growth of tissues and repair of normal damage require cells to move in precisely-controlled ways. Immune responses require cells that migrate through tissues and into structures like lymph oneds to meet and exchange information. And disease processes often use cell migration. Perhaps the best-known case is cancer metastasis. Cancer cells may start to migrate away from their original tumour. In doing so they enter the bloodstream or lymph vessels, and spread to other sites, underpinning much of the damage caused by cancer. Cells crawl using similar structures called lamellipods and pseudopods. These protrude from the front of the cell, engage with the local environment and adhere, then provide the framework for cells to pull on and drive themselves forwards. They are made of a small protein called actin, and their formation and maintenance are controlled by a large protein assembly called the Scar/WAVE complex, or WRC. In this grant we seek to understand how the WRC works. If we can understand this, we can understand how cells choose whether to move, and which direction they go in when they do so. We want to understand the mechanisms through which cells convert the inactive WRC to the active form. The inactive form resides in the cytoplasm and interacts with few other proteins. The active form behaves completely differently - it seems to act as a hub by binding, localising and activating many the proteins that make lamellipods and pseudopods. In this grant we seek answers to the following questions: 1. What is the most important property of active WRC? Is there one protein that is particularly crucial, and if so what is it? And if not, does it just act as a general hub, pulling together many of the proteins a cell needs to move? 2. If we reveal the molecular structure of the WRC, using the advanced technique of cryo-electron microscopy, can we understand how this change from inactive to active forms is orchestrated? Our preliminary data show that current ideas for how this works are probably wrong. 3. Given a structure for WRC - can we identify what happens in the cell to control the activation of the WRC? And 4. Can we find out how the active WRC signal is turned off? A full answer to these four questions would cause a complete refocus in how we understand cell migration, and inform every biomedical scientist who works on cells that move.