UTas Biophysics Lab
MIFE Research Facility
MIFE user group
Ion Fluxes and Membrane Transport - Background
Membranes constitute a barrier to free diffusion of molecules and underlie many essential cell biological processes including nutrient acquisition and compartmentation, pH and ionic homeostasis, turgor generation, metabolite distribution and waste excretion, energy transduction and signaling. According to Ward (2001), 43% of over 25,000 protein sequences in the Arabidopsis genome have at least one transmembrane spanning (TMS) domain with 18% proteins having two or more TMS domains and are thus associated with cellular membranes. Recent progress in electrophysiology and molecular genetics has revealed the crucial role of plasma membrane transporters in perception and signaling in response to virtually every known environmental factor (Zimmermann et al. 1999). In many organisms, changes in plasma membrane potential or modulation of ion flux are amongst the earliest cellular events in response to light, temperature, osmotic stress, salinity, hormonal stimuli, elicitors and mechanical stimulation. For many, if not for all the stresses mentioned, the receptors involved were suggested to be located at one of the cellular membranes. In addition to hosting various receptors mediating plant-environment interactions, membrane transporters always act as the ultimate effectors, enabling plant adaptive responses. Such a central role of membranes and membrane transport processes in cellular adaptive responses to environmental conditions makes them important targets for genetic manipulations aimed to improve tolerance to a particular stress. To enable this, causal links between membrane-transport processes and other metabolic or physiological processes in the cell need to be understood.
Gaining such an understanding is not an easy task. It is complicated not only by the large number of transporters involved (for cations, 46 unique families are known, containing approximately 880 members in Arabidopsis; Maser et al. 2001), but also by the myriad of interactions and communication between various transporters and signaling components. Over the last three decades, various state-of-the-art molecular and biophysical techniques (such as patch-clamp or fluorescence imaging) have been used to reveal some of these interactions. However, at the same time, the inevitable consequence of such invasive approaches was a decrease in the physiological reality of the transportersí environment (Tester 1997). There are many reports showing that activity of a particular transporter differs dramatically when expressed in a heterologous system compared with in situ conditions. This makes it very difficult [and often even impossible] to transfer the results obtained by these advanced techniques to real living systems in their natural habitats. The more advanced our study, the bigger is the gap between physiologists/molecular biologists and the agronomists interested in plant behavior in the field.Since 1990 the UTas Biophysics and Agricultural Science laboratories have pioneered the application of the MIFE system as a non-invasive microelectrode ion flux measuring technique in plant nutrient transport and stress physiology, recently broadening it to other living systems including bacteria, fungi, yeasts, animal and mammalian cells. Selected examples of this work are available in our reprints. Some of the key features of the MIFE system (non-invasiveness, high spatial and temporal resolution etc) make it an ideal tool to establish and quantify causal links between membrane-transport processes and other metabolic or physiological processes in the cell.
Maintained by Ian Newman. Date . © University of Tasmania.