Drosophila researchers are notoriously resourceful when it comes to finding ways of extracting functional information from the fruitfly genome. By manipulating the transposable P-element, for instance, they have gained fast access to mutant phenotypes through insertional mutagenesis, and to gene expression profiles through enhancer trapping — in which a promoter-less reporter carried by a P-element reveals the expression pattern of an endogenous gene when 'captured' by the gene's enhancer. But these methods fail to inform about the behaviour of the protein expressed by the trapped gene — will it travel to the plasma membrane, rest on an organelle or degrade after a few minutes? Morin and colleagues have engineered a gene-trap vector that provides just such information: by tagging genes with a GFP molecule, they can identify the whereabouts of the encoded fusion product in living cells. Unlike antibody staining, which provides similar information, GFP protein tagging doesn't require any knowledge of the protein itself — it is quicker, can be done in living cells and, as the authors show, can bring to light (literally!) previously unknown genes.

The ability of the construct designed by Morin et al. to tag proteins requires flanking a GFP reporter gene with a splice acceptor and splice donor site, so that, when integrated into an intron, this foreign exon can be spliced between the amino- and carboxy-termini of a mature protein. The GFP construct lies within a P-element, by which it is delivered to flies and is mobilized to many random positions in the genome. In the more than 600 GFP-expressing lines of flies that the authors recovered, fluorescent proteins were collectively seen in virtually every cell compartment and, where known, their expression pattern faithfully recapitulated those of endogenous proteins, apparently without ill-effect on normal mRNA splicing or on protein folding. Curious to know which genes they had trapped in their screen, the authors sequenced the genomic DNA flanking 102 insertions; however, just under half did not match known or predicted genes — a sign perhaps that protein trapping can pick up unconventionally structured ORFs.

Although powerful, this technique does have its limitations. One intrinsic disadvantage is the insertion specificity of the P-element, which is biased towards genes with larger introns. Another one is the failure of the human eye to detect weak GFP signals — a drawback that an automated sorter can overcome. With more emphasis being given to understanding development in real time, the in vivo GFP protein markers created in this study will no doubt complement existing methods in flies for studying the dynamics of gene expression and cellular behaviour.