Proline-rich regions in transcriptional complexes: heading in many directions.

Although many protein complexes are now known, the precise details of how each component operates are rarely understood. Through a combination of bioinformatics and analysis of gene-trapped mouse clones, Enkhmandakh et al. were able to deduce the modular function for a part of Ssdp1, a crucial component of the Ldb1 transcriptional complex, which plays a central role in mammalian head development. A proline-rich module from Ssdp1 is likely responsible for transactivation, and this region is curiously mobile, occurring in different proteins in different species. The results underscore the importance of protein modularity in complex organization, as well as the role of irregular or disordered parts of proteins in crucial biological processes.