Co-operating kinesins: understanding redundancy in microtubule motor systems

Cells contain a system of filaments, called microtubules, that are made up of polymers of a protein call tubulin. Microtubules act as tracks within the cell for the transport of small structures from one region to another. This delivery system relies on proteins that 'walk' along microtubules, acting as minute motors. These motors can be attached to a variety of different structures in the cell, or 'cargoes', in much the same way that a railway engine can pull passenger coaches or freight wagons. We are interested in a group of related motors, called the kinesin family. Three sub-families (kinesins-1, -2 and -3) are known to drive the movement of membrane cargoes, which are tiny bags made up of lipid and protein that act as a delivery system for new material within the cell. The need for this active transport is particularly clear in nerve cells, where membrane vesicles carrying components needed for passing messages from one nerve cell to the next need to be delivered over long distances of up to a metre in our bodies. Even with this active transport system, this journey will take 2.5 days for material made in your spinal cord to be carried all the way down a single nerve cell to your big toe. If kinesins do not function properly, perhaps because they have a mutation, this leads to severe traffic jams, and is known to cause motor neuron disease. Most cells in the body are much smaller than nerve cells, however, and even though they contain kinesin-1, it is hard to see any effect if its motor activity is blocked. This could be because more than one type of motor co-operates to drive the movement of any particular cargo. We have evidence that this is the case for a particular cellular membrane structure, the Golgi apparatus. We will test the role of three types of kinesin motors in the movement of these membranes, and also in the transport of the tumour suppressor protein Adenomatous Polyposis Coli protein (APC). We will also establish how kinesin-1 can bind to APC and transport it to the edge of the cell. This work will define how motors work together to transport cargoes.

Kinesin family members drive the movement of many different cargoes, and these transport events are crucial for many subcellular processes. While loss of kinesin-1 function has severe consequences in large cells such as neurons, inhibiting kinesin-1 has only subtle effects in small cells. This could be due to multiple kinesins performing overlapping roles. We will test the hypothesis that members of the kinesin-1, -2 and -3 families co-operate to drive the movement of two different cargoes. In support of this model, we have found that disruption of these motors individually has little effect on the morphology and distribution of the Golgi apparatus, while inhibiting the motors in pairs causes a profound fragmentation and scattering of the Golgi apparatus. Using a combination of targeted and systematic inhibition of motor function, we will determine the which kinesin-1, -2 and -3 family members drive the movement of membranous cargoes in the early secretory pathway. In addition, we will investigate why the inhibition of plus end-directed microtubule motors should generate a phenotype identical to that seen when a minus end-directed motor, dynein, is inhibited. We will also test the same hypothesis for the movement of a non-membranous cargo, the tumour suppressor Adenomatous Polyposis Coli protein (APC), since members of the same three classes of kinesin have been implicated in its transport. In addition, as we have preliminary data that suggest that kinesin-1 can interact with APC, we will determine the mechanism of interaction. This work will define for the first time the full complement of motor proteins that drive cargo movement in two specific cases. It will also lead to a better understanding of the complex motility of membranes in the early secretory pathway, and of APC delivery to the leading edge of migrating cells.