Symportin Synchrony: Ribosomes, the macromolecular machines responsible for protein synthesis, function in the cytoplasm but are assembled in the nucleus. Ribosomal proteins must be imported into the nucleus, but how this is coordinated with assembly is unclear. Kressler et al. (p. 666 ) report that two 5S rRNA binding proteins are coimported into the nucleus. They identify a transport adaptor, which they term symportin (Syo1), that binds simultaneously to Rpl5 and Rpl11. Syo1 also interacts with the import receptor Kap104, which facilitates import of the Syo1-Rpl5-Rpl11 complex. Synchronous nuclear transport may be more generally used to coordinate assembly processes.

Flagella are the bacterial organelles of motility and can play important roles in pathogenesis. Flagella biosynthesis requires the coordinated export of huge protein amounts from the cytosol to the nascent flagellar structure at the cell surface and employs a type III secretion system (T3SS). Here we show that the integral membrane protein FlhA from the gram-positive bacterium Bacillus subtilis acts as an adaptor for late export substrates at the T3SS. The major filament protein (flagellin) and the filament-cap protein (FliD) bind to the FlhA cytoplasmic domain (FlhA-C) only in complex with their cognate chaperones (FliS and FliT). To understand the molecular details of these interactions we determined the FlhA-C crystal structure at 2.3 Å resolution. FlhA-C consists of an N-terminal linker region, three subdomains with a novel fold, and a disordered region essential for the adaptor function. We show that the export protein FliJ associates with the linker region and modulates the binding properties of FlhA-C. While the interaction of FliD/FliT is enhanced, flagellin/FliS is not affected. FliJ also keeps FliT associated with FlhA-C and excess of FliT inhibits binding of FliD/FliT, suggesting that empty FliT chaperones stay associated with FliJ after export of FliD. Taken together, these results allow to propose a model that explains how the T3SS may switch from the stoichiometric export of FliD to the high-throughput secretion of flagellin.

Protein targeting by the signal recognition particle (SRP) is universally conserved and starts with the recognition of a signal sequence in the context of a translating ribosome. SRP54 and FtsY, two multidomain proteins with guanosine triphosphatase (GTPase) activity, are the central elements of the SRP system. They have to coordinate the presence of a signal sequence with the presence of a vacant translocation channel in the membrane. For coordination the two GTPases form a unique, nearly symmetric heterodimeric complex in which the activation of GTP hydrolysis plays a key role for membrane insertion of substrate proteins. Recent results are integrated in an updated perception of the order of events in SRP-mediated protein targeting.

Docking of cytoplasmic and membrane receptors facilitates conformational changes that drive protein insertion.