Dielectric spectroscopy, because of its non-invasive nature, has recently been tabbed as a realistic method for the tag-free detection of proteins within biological cells. Well equipped to test the viability of this notion, we have embarked upon a series of experiments to determine if the presence of proteins within living cells is evident from dielectric measurements. The first of these investigations combined the techniques of fluorescence imaging, a classical in vivo protein detection method, and dielectric spectroscopy in order to characterize the contributions of proteins to the dielectric properties of the plasma membrane1. In this experiment, two types of cellular suspensions were prepared, one consisting of yeast cells genetically engineered to overexpress a membrane receptor (the sterile2-α factor receptor protein, Ste2p) fused to the green fluorescent protein, and another consisting of control cells lacking the Ste2p. The dielectric measurements were analyzed using the spherical two-shell model 2 to determine the plasma membrane permittivity of the cells of each of the samples, the makeup of the plasma membrane being the only difference between the two populations of cells.
The results indicated a slight difference in the value of the plasma membrane permittivity between the two samples, lying just at the limit of statistical insignificance. Information obtained from the fluorescence measurements confirmed that not all cells in the Ste2p-expressing suspension kept the plasmid and synthesized Ste2p, which means that the detected plasma membrane permittivity was an average over two cell types contained by the sample that was engineered to express Ste2p: those that expressed and those that failed to express Ste2p. This means that the observed differences would have been more dramatic if all cells in our samples expressed Ste2p. Motivated by these results, the next line of experiments will involve sorting and separating the Ste2p-expressing cells, allowing us to avoid the previously mentioned averaging effect and thereby accurately determine their plasma membrane properties.
1. Stoneman M, Chaturvedi A, Jansma D B, Kosempa M, Zeng C and Raicu V 2007 Protein influence on the plasma membrane dielectric properties: In vivo study utilizing dielectric spectroscopy and fluorescence microscopy Bioelectrochem.70 542-550
2. Raicu V, Raicu G and Turcu G 1996 Dielectric properties of yeast cells as simulated by the two shell model Biochim. Biophys. Acta-Bioenerg. 1274 143-8