Imaging of metabolites by using a fusion protein between a periplasmic binding protein and GFP derivatives: From a chimera to a view of reality

Biological systems contain an immense number of individual components, which undergo dynamic and highly interactive responses in time and space. Analysis of these responses will provide a key to unlock the information encrypted in the genome sequences that are accumulating around the world. This task is being driven by powerful methods that allow comprehensive of gene expression, protein localization and protein–protein interactions. Emerging technologies that allow a comprehensive analysis of metabolites (1) will also make a vital contribution, by uncovering many of the phenotypic changes that result from alterations of the genotype, or that accompany changes in gene expression. High throughput profiling technologies suffer, however, from a serious blind spot. Understanding of biological function also requires spatial resolution, at the cellular and subcellular level. For several decades, this sort of information was obtained in a painstaking way, for example, by isolating cell types or organelles and investigating what proteins, enzyme activities, and metabolites they contained (2), or by producing tissue sections for in situ hybridization or immunolocalization of transcripts and proteins. Molecular cell biology now uses generic methods to investigate the tissue and cell-specific localization of transcripts and the cellular and subcellular distribution of proteins, for example reporter genes or GFP fusion proteins. Analogous techniques are urgently needed to measure metabolite levels in situ. Metabolites change even more dynamically than transcripts or proteins, but only a minute fraction of metabolites possess spectral properties that allow them to be directly visualized and imaging by NMR is restricted to metabolites that are present at relatively high concentrations (3). The development of generic techniques to monitor in situ metabolite levels is an enormous technical challenge, because of their immense chemical heterogeneity.

In this issue of PNAS, Fehr et al. (4) take an important step toward this goal. By using a strategy that …