Tuesday, 18 July 2017

Highlights from the summer 2017 ISMB seminar programme

Regular readers of this blog will know that the Institute of Structural and Molecular Biology (ISMB) coordinates the research efforts in the Department of Biological Sciences at Birkbeck and two departments – Chemistry and Structural and Molecular Biology – at neighbouring University College London. Research in the participating departments is coordinated through six core research centres, and many grants, PhD studentships and experimental facilities are held in common.

Since 2010, too, these departments’ seminar programmes have been consolidated into termly series of ISMB seminars, giving the Institute’s researchers and students the chance to hear world-class scientists present their work. Most of each term’s seminars are centred round a theme, with recent themes including bioinformatics; the molecular basis of infectious disease; and, in the most recent series, ‘beyond signalling’. This post briefly describes two seminars in this series, both from researchers based in London and both closely concerned with disease mechanisms.

Paul Freemont holds a chair in protein crystallography at Imperial College, London. His group has solved the structure of several proteins linked to cancer, including a domain called the ‘RING finger’ that is found in the breast cancer susceptibility protein BRCA1. His talk to the ISMB, however, concerned a research interest that he shares with the Institute’s head, Gabriel Waksman: the membrane-bound protein complexes through which Gram negative bacteria secrete toxins and other molecules across the double bacterial cell wall and out of the cell. These bacteria have evolved at least six such ‘molecular machines’, with yet another found in mycobacteria such as M. tuberculosis. Waksman’s work in elucidating the structure of the Type IV system is covered extensively in section 11 of the PPS course (‘Structures of Membrane Proteins’).

Freemont’s seminar described the Type VI secretion system (T6SS), which was first identified as recently as 2006 in Vibrio cholerae (as its name suggests, the causative agent of cholera). The function of the T6SS is to eject proteins from the interior of the bacterial cell into an adjacent cell, which may be either bacterial or eukaryotic. Freemont’s lab mainly studies these systems in the bacterium Pseudomonas aeruginosa, an opportunistic pathogen that causes infections mainly in people who are already chronically ill, such as cystic fibrosis patients and those with severe burns.

This secretion system has been described as a ‘molecular syringe’. Its structure resembles that of the tail of a bacteriophage – a type of virus that affects bacteria – but it is inverted, with the tip of the tail pointing away from the bacterial cell wall and towards its target cells. In some species, the same secretion systems can target both eukaryotic cells and other bacteria. The system consists of a long sheathed tube, built up from many protein subunits, that is large enough to be easily viewed using electron tomography and that is tipped by a spike through which the protein to be delivered is ejected. Energy for cargo delivery is provided by the contraction of the tube, with a single contraction storing the energy equivalent of 1600 molecules of ATP. The whole structure is dynamic; it is assembled only when needed and disassembled after the cargo has been delivered, allowing the cycle to begin again.

Although the T6SS can sometimes act as a cell-to-cell ‘killing machine’, as in Vibrio cholerae, protein delivery to the target cell will often have rather more subtle effects, with Pseudomonas aeruginosa a case in point. This rod-shaped Gram negative pathogen uses three distinctly different type 6 systems, encoded on separate operons, to secrete effector proteins that interfere with the host immune system. Freemont and his group have solved the structures of several P. aeruginosa T6SS components using X-ray crystallography, throwing more light on their phage-like mechanism of action. Structures of an accessory protein (TagJ) and the ATPase that catalyses sheath disassembly (ClpV) were all published in the Journal of Biological Chemistry in 2014; some other component structures are yet to be published. TagJ is now known to interact both with ClpV, an AAA+ ATPase, and with components of the sheath, and this interaction allows the rapid disassembly that is required for the complete system to be reset. Each ATPase only interacts with the components from its own operon. Further structural studies, using high-resolution electron microscopy as well as X-ray crystallography, are expected to elucidate further details of these complex molecular machines and to suggest ways in which they might one day be targeted by the novel antimicrobial drugs that we so desperately need.

The second London-based seminar speaker, Miriam Dwek from the University of Westminster, had a somewhat unorthodox beginning to her research career at Oxford University’s first spin-off company, Oxford Glycosystems (now, after many mergers, part of pharma giant UCB). She has maintained her interests in glycobiology (the biology and biochemistry of sugars and polysaccharides) and its application to human disease – particularly breast cancer – into and throughout her academic research career.

Breast cancer is one of the most common cancer types, with 400,000 new cases occurring each year in Europe alone. Breast tumours can be divided into many subtypes with different genetic and biochemical profiles; although some are now easily treated with surgery, radiotherapy and/or drugs, others are often fatal (if perhaps after many years). Generally speaking, tumours are tractable when they are confined to breast tissue and the disease only becomes difficult to treat once it has spread. All cancer types metastasise in a particular pattern; breast cancers tend to spread first into nearby lymph nodes and then to the lungs, brain or bones.

Metastasis is a complex, multi-step process, and selecting the optimum treatment for each patient depends on detecting whether and how her tumour will metastasise as early as possible. Changes in the concentration of some biological molecules in body fluids have been associated with tumour growth and development, and these biomarkers can be used as easily-measurable surrogates of cancer development. One particularly well-known example is the prostate-specific antigen (PSA), a glycoprotein found in semen that is elevated in prostate cancer. No such clear-cut examples exist in breast cancer, but many subtler biochemical changes are known to occur. Dwek and her group have been exploring differences in protein glycosylation patterns between breast tumours and normal breast cells.

Glycosylation is one of the most common post-translational modifications of amino acids (link is to PPS section 2). There are two basic types; one or (almost always) more monosaccharides can be bonded to the oxygen atoms of serine and threonine side chains (O-glycosylation) or to asparagine’s side chain nitrogen (N-glycosylation). The addition of the first residue to the amino acid and the subsequent extension of the chain are catalysed by enzymes in the transferase class. In O-glycosylation, in particular, the patterns of residues added to the glycan ‘branches’ differ between healthy breast epithelial cells and breast tumour cells, and this, in turn, can aid the process of cell adhesion (binding cells together), which is essential for tumour metastasis. Cadherins are glycoproteins that have important roles in cell adhesion, and Dwek’s group used glycoproteomics techniques to identify this as a potential biomarker of metastatic breast cancer. She considers that it is likely to be particularly useful for detecting metastasis in patients with estrogen receptor positive tumours and vascular invasion.

Other topics presented by leading researchers in this ISMB seminar series included nuclear receptors, collagen-binding proteins and protein targeting and translocation. The seminar programme will return in October, and I will doubtless be returning to it again in this blog.

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