Rab proteins, microtubule motors and the organisation of the endocytic pathway

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The behaviour of cells within a tissue is controlled by their environment. Amongst the most important signals that cells receive are in the form of circulating small proteins called growth factors. These bind to specific receptors that are found on the surface of cells. Binding of growth factors causes the receptors to alter their pattern of interactions with many molecules inside the cell that control cell growth. In this way growth factor receptors act as essential bridges between the cell exterior and interior to stimulate proliferative, or mitogenic responses. In order to prevent such mitogenic responses continuing endlessly, which would lead to uncontrolled cell division, the growth factor receptor must be inactivated shortly after the growth factor binds. This is achieved by removing the activated receptor from the cell surface and sending it to a specialised compartment within the cell, where it can be degraded. This compartment is called the lysosome. Movement of the receptor from the cell surface to the lysosome involves the receptor being sequestered into regions of the cell surface membrane that invaginate and pinch off to form spherical packages, or vesicles, within the cell interior. These vesicles move in a directed fashion to the lysosome, via a number of intermediate compartments. The situation becomes more complicated when it is realised that the route that growth factor receptors take to the lysosome is also followed part of the way by other types of receptor, which are engaged in taking up nutrients. These receptors are returned to the cell surface from the intermediate compartments so that they can participate in further rounds of nutrient uptake. Hence, at a critical point along the pathway towards the lysosome, these receptors are selected away. The aim of this project is to understand how this diversion takes place.