, 2009). The mechanistic links now uncovered by Yoon et al. illustrate how the study of local translation not only can benefit our understanding of this widespread posttranscriptional regulatory mechanism, but can also help more generally to uncover unexpected functions of molecules BVD523 within specific compartments

of the cell. Yoon et al. (2012) came across this unsuspected role of lamin B2 in a proteome-wide screen for proteins synthesized in axons in response to the extracellular cue Engrailed. Engrailed is a homeodomain protein, long known as a nuclear transcription factor. Although at first sight Engrailed might not seem like an obvious molecule to use as an extracellular cue, work most notably by the group of Alain Prochiantz has shown that homeodomain proteins can cross the cell membrane, and previous studies by the Prochiantz and Holt groups showed that Engrailed can act as a guidance cue for retinal ganglion cell (RGC) axons

(Brunet et al., 2005). These are the axons that transmit information from the retina to the tectum, the primary visual center of the brain in nonmammalian vertebrates. Connections between the retina and the tectum are highly organized topographically to produce an accurate representation of the outside world in the tectum. To generate these orderly connections during development, RGC axons are guided within the tectum by gradients of cues, including ephrins and Engrailed (Luo Carfilzomib supplier and Flanagan, 2007). Yoon et al. (2012) chose to study RGC axons and Engrailed

because the turning response is translation dependent, and Engrailed strongly upregulates axonal protein synthesis. To screen for proteins synthesized in axons after Engrailed stimulation, Yoon et al. (2012) ingeniously combined a metabolic labeling technique with 2D gel analysis. Axons were isolated in culture, stimulated with Engrailed and newly synthesized proteins labeled by incorporation of a modified amino acid (AHA) that can be subsequently fluorescently tagged (Dieterich et al., 2010). Newly synthesized proteins from Engrailed stimulated and unstimulated axons were labeled with differently colored fluorophores and run together on a 2D gel, where proteins whose synthesis Electron transport chain was upregulated, downregulated, or unchanged could be identified as green, red or yellow spots respectively. By mass spectrometry of these protein spots, they identified twelve proteins increased by Engrailed in the axon, and surprisingly lamin B2, a protein known for its nuclear functions, was induced the most strongly (Figure 1). Extraordinary claims tend to require extraordinary evidence and Yoon et al. (2012) used an impressive series of experiments to provide evidence that lamin B2 is synthesized and functions within the axon.