Monday, 9 March 2009

Structure of the trypanosome microtubule cytoskeleton

Twice a term, the School of Crystallography hosts a seminar for the whole of the Institute of Structural and Molecular Biology, which consists of research departments in related disciplines from both Birkbeck and neighbouring University College London. Tom Blundell's (see previous post) was an ISMB seminar; today, we heard one from Professor Keith Gull of the Sir William Gunn School of Pathology, University of Oxford.

Much of Keith Gull's work concerns the single-celled parasite Trypanosoma brucei, which is endemic in large parts of Africa and causes African trypanosomiasis, otherwise known as sleeping sickness. Diseases like this one attract relatively little research funding even though they are important causes of morbidity and mortality in many poor countries; they are classed as neglected tropical diseases. Keith and his colleagues in Oxford are studying many aspects of the molecular biology and genetics of this parasite.

The trypanosome cell surface is covered by a dense coat made up of very many copies of a single protein, called variable surface glycoprotein (VSG). A glycoprotein is a protein with carbohydrate (sugar) groups attached to one or more amino acid sidechains; the word "variable" is used because there are about 1000 variants of this protein. Each of these variants is encoded by a different gene, so the VSG genes account for about 10% of the trypanosome's genome. However, only one of these is expressed at any time, so the trypanosome coat (almost) always consists of multiple copies of a single protein. (There will be times when two proteins are present because the expressed variant is in the process of changing.) Although the sequences of VSG proteins differ considerably, their structures are very similar: they are anchored to the cell membrane and the large extracellular domain consists of an antiparallel coiled coil of alpha helices (see PDB file 2vsg).

Most of Keith's talk, however, concerned the structure of the trypanosome cytoskeleton, which consists of microtubules. Very basically, these are composed of polymers of a protein called tubulin and they are involved in maintaining the structure of components of many types of cells. Keith and his group have used the technique of electron tomography - a type of transmission electron microscopy - to obtain exquisite pictures - if at much lower than atomic resolution - of the structure of this cytoskeleton and begin to understand its function and role in trypanosomal cell division. Structures of these multi-protein complexes are not - yet - available in the PDB.

Trypanosomes have flagella - whip-like structures attached to the cells that can propel them through the host bloodstream. Flagella are found in many cell types, and if they have receptors bound they can also be used to sense cells' surroundings. The trypanosome flagellum is attached to the cell body via a filament and this point of attachment is within a pocket that, interestingly, is the end point for all vesicular traffic within the cell. This means that this pocket needs to contain a large number of proteins vital to the parasite's survival, including its transferrin receptor and haemoglobin receptor. Imaging the structure of the trypanosome cytoskeleton, particularly in the pocket where the flagellum attaches, has also given insight into the rather unusual process through which this cell conducts mitosis (cell division).