Taking the inventory inside single cells.

developed to measure cell-to-cell differences have typically been based on microscopic imaging of fluorescent proteins 1 or RNA 2,3. These techniques make it possible to monitor RNA or protein fluctuations both in single cells over time and across the cell population , and they are even capable of visualizing single molecules in single cells 4,5. However, they can only be used to monitor a few types of molecules (for example, ones selectively labeled) in relatively stable conditions. A technique recently introduced by Huang et al. 6 enables one to measure a larger number of molecular species in changing environmental conditions. This new approach involves all the steps typical to a gel assay, but the 'laboratory bench' is squeezed down to the impressive size of a square inch (Fig. 1). Cells are pumped into a microfluidic channel and captured one by one in micrometer-sized reaction chambers. The addition of a buffer solution causes cell lysis, which can be followed by fluorescent labeling of the resulting protein mass, if necessary. The various protein species are separated by capillary electrophoresis and quantified with the clever use of cylindrical optics and fluo-rescence microscopy. Various molecular species can be measured individually, molecule by molecule, and therefore it is possible to capture proteins that are low in copy number within cells. Importantly, capillary electropho-resis separates proteins based on their charge and molecular weight (Fig. 1). This eliminates the need of tagging different molecular species with different fluorescent labels, thereby avoiding the potential adverse effects that tags can have on protein expression. For similar reasons , this approach can also be readily applied to organisms for which genetic manipulation is difficult, if not impossible. To test their method, Huang et al. 6 measured the expression of a human transmembrane protein expressed in an insect cell line. They also examined the degradation of phycobili-some protein complex components that takes place when the cyanobacterium Synecococcus sp. PCC 7942 is transferred from a nitrogen-rich environment to a nitrogen-depleted environment (in the absence of external nutrients such as nitrogen, Synecococcus spp. degrade their phycobilisomes in an ordered manner and use them as nutrients). The advantage of the strategy of electrophoretically separating the components before analysis is particularly clear in the second case: Huang et al. quantified phycobilisome components with highly overlapping fluorescence spectra in single cells, which could not have been measured by microscopy. They observed a high variability of protein …