Membrane proteins are abundant, comprising approximately 30% of all gene sequences. Traditionally, elucidating the structure of membrane proteins has been fraught with difficulties due to their lipid-embedded domains. This is illustrated by the fact that there are less than 50 unique structures of membrane proteins solved to atomic resolution, compared to 3000 unique crystal structures of soluble proteins.
The aim of this project is to examine the molecular structure of the CLIC1 chloride ion channel within its native membrane environment, using atomic force microscopy. Dr Stella Valenzuela was responsible for the cloning and characterisation of CLIC1, a novel nuclear chloride ion channel which represents the first human member of a new class of intracellular chloride ion channel proteins. The unique feature of these proteins is that they function as both soluble and integral membrane proteins. This work was published in Journal of Biological Chemistry in 1997. She then went on to show in 2000 that CLIC1 is ubiquitously expressed and is involved in regulation of the cell cycle. The further importance of CLIC1 is highlighted by its wide distribution through most, if not all, human tissues studied. CLIC1 has been shown to insert into artificial lipid bilayers and phospholipid vesicles with similar electrophysiological properties to those observed in vivo. As part of a collaborative team she has recently published x-ray crystallography data determining the soluble form of CLIC1, but the membrane channel structure remains unknown and is difficult to determine using traditional atomic resolution structural techniques (for example crystallography or NMR). Nevertheless, elucidating CLIC1 structure and behaviour within a membrane environment is imperative to understanding its physiological function in cells.
The task is being tackled at UTS by Dr Valenzuela and Mr Mark Berkahn of the MAU, using a specialised 'wet cell' fitted to a high resolution AFM. Molecular-scale resolution of both lipid surfaces and ion-channel implanted lipid surfaces have been achieved, suggesting that the attainment of the ultimate goal of this project is close.
