Viewed through a microscope, cells undergo a spectacular transformation as they enter mitosis, the phase of their existence just before they divide. The formation of the mitotic spindle, where the cell?s network of microtubule fibres is completely rearranged to span from either end to the chromosomes at the centre, is particularly striking. This spindle is a molecular machine that ensures the cell?s chromosomes are accurately distributed between its two daughter cells. Errors in the workings of the spindle are a known driving force of cancer and are also responsible for a congenital brain disease. Several control mechanisms ensure the mitotic spindle is normally assembled correctly. At an early stage in assembly, two proteins called TACC3 and ChTOG promote microtubule stability and hence promote assembly. These proteins are more effective when TACC3 is modified by a phosphate group: one phosphorous atom and three oxygen atoms that is commonly used by cells to alter the activity of their proteins. In the case of TACC3, the protein that adds the phosphate is called Aurora-A. Spindle assembly is thus controlled by the activity of Aurora-A, which is itself controlled by many other proteins under the influence of events within and outside the cell. We propose to investigate how the phosphate group influences the effectiveness of the TACC3/ChTOG partnership at the level of atoms. How this works is currently a mystery as the phosphate is only four atoms big, and yet it changes the activity of TACC3/ChTOG which total tens of thousands of atoms. We will use electron microscopy, a technique that allows us to see directly the shapes of proteins, to study the changes in TACC3 upon phosphorylation, and the effect on ChTOG. We will also use X-ray crystallography to determine the location of every atom within the proteins and to map the atoms by which TACC3 and ChTOG cooperate. This information will allow us to make a hypothesis for the details of how the TACC3/ChTOG partnership works and how phosphorylation enhances their effectiveness. We will use our protein structure models to design subtle modifications to TACC3 and ChTOG to test this hypothesis in human cells grown in culture. An overabundance of TACC3, ChTOG or Aurora-A have been linked with cancer, and TACC3 and Aurora-A are also important in brain development. These studies will provide the impetus for future investigations to understand the role of these proteins in human disease.