Research Goals:

Why membrane proteins?

The structures of very few membrane proteins have been determined, but these proteins are targets for over half the current FDA-approved drugs. Hence there are huge gaps in our knowledge concerning this group of proteins, and large bridges to cross before we can enter the domain of wealth generation . As explained elsewhere, structural proteomics is unlikely to make large inroads into this area in the near future. The membrane proteins of complex compartmentalised and multi-celled organisms such as humans (ie eukaryotic membrane proteins) are even more challenging for the structural biologist than ones from bacteria, but the research community is quite close to breaking through this barrier. The structural biology of eukaryotic membrane proteins will become the main area for wealth generation.

Why ABC transporters?

1. ABC proteins are a very large family of membrane proteins, and therefore understanding their structure/function relationships will have a major impact on our understanding of the biology of the cell. For example, sequenced genomes display a large number of known or putative ABC proteins: Plasmodium falciparum = 13 genes, Arabidopsis thaliana = ~129 genes, Anabaena sp. = ~150 genes, Yeast = ~ 31 genes, Human = 48 genes, Drosophila melanogaster = 56 genes, Caenorhabditis elegans = 56 genes. It is significant that a parasite (Plasmodium falciparum) has the fewest ABC genes, whilst the primary producers (Arabidopsis and Anabaena) have the most.

2. Important human diseases involve ABC proteins, and therefore structural analysis of these proteins may lead to better understanding of the nature of the diseases and/or the development of novel drugs. Cystic fibrosis is probably the most significant human disease involving an ABC protein - CFTR. Over 1000 different mutations in this protein are known to lead to this debilitating defect. Drugs that increase CFTR activity or promote the maturation of CFTR are sought.

   Multi-drug resistance of cancer cells is linked to the frequent failure of chemotherapy. This disease process is also associated with at least 3 ABC proteins in humans (P-gp, MRP1, BRCP), although in this case the disease is associated with an overexpression of the protein function, and hence drugs to inhibit these proteins are sought. Other human diseases associated with other ABC proteins are listed below:

   Bare lymphocyte syndrome (immunodeficiency). Tangier disease (high density lipoprotein deficiency). Stargardt disease (macular degeneration). Anaemia/ataxia (mitochondial ABC defect). Cholestasis (bile disease/liver). Dubin-Johnson (liver). PXE(Pseudoxanthoma elasticum) -intestinal haemorrhage. Hypoglycemia/Hypoinsulinism (Sulfonylurea receptor gene). Adrenoleukodystrophy, ALD (brain). Sitosterolemia (sterol uptake defect).

3. ABC proteins in pathogens are potential drug targets: By knocking out ABC proteins in pathogens, novel therapies and antibiotics may be developed. Examples of ABC proteins involved in uptake and efflux in pathogens are listed below: Uptake: e.g. Yfe, YbtQ, YbtP (Yersinia pestis) - Fe and Mn uptake; Adc and PsaA S. pneumoniae - Zn and Mn uptake; SitABCD (Salmonella enterica)- Mn transport. Efflux: e.g. LmrA multi-drug exporter in Lactococcus lactis;

4. ABC proteins function by coupling ATP binding and hydrolysis to a large conformational change in the protein.This conformational change gives rise to a range of different functions. It can act as a means of transport (in either direction – efflux or uptake) as in P-glycoprotein. It can act as a flippase (moving substances from one leaflet of the lipid bilayer to the other) as in MsbA. It can act as a signal, triggering a series of downstream events via interactions with other proteins (as in the sulfonylurea receptor). It can act as a gating mechanism for a channel opening and closing (as in CFTR). Given the huge range of ABC proteins, it seems certain that further adaptations of the conformational change will be identified. The conformational change is therefore adapted to a range of tasks in different ABC proteins, which makes this process particularly interesting from a nanotechnology perspective,. In a conceptual sense, the ABC protein family represent an example of ideal nanomachines, utilising a common power source and motor to perform a range of functions that can be dictated by fine-tuning of the primary amino acid sequence. An understanding how this is arranged in these proteins therefore represents an opportunity for conceptualising the design and operation of future nano-devices.