Keller et al. describe how two transport pathways cooperate to insert a bacterial protein into the cell membrane.
Bacteria and chloroplasts have two different systems that translocate proteins across or into membranes. The Sec pathway transports unfolded proteins through the SecYEG membrane channel, whereas the twin-arginine transport (Tat) machinery translocates proteins such as the Rieske redox protein, whose C-terminal iron-sulphur domain must be carefully folded in the cytosol before being transported across the membrane. The Rieske proteins of Streptomyces coelicolor and other actinobacteria, however, contain three transmembrane domains (TMDs) instead of one and have N termini that lack the twin-arginine motif usually recognized by the Tat machinery. How these bacteria insert their Rieske proteins into membranes is therefore unclear.
Keller et al. identified an internal twin-arginine motif next to the third TMD of the S. coelicolor Rieske protein Sco2149. Mutating these arginines, or deleting the Tat machinery, blocked membrane integration of the third TMD of a Sco2149-based reporter expressed in E. coli. The first two TMDs were still inserted successfully, however. Keller et al. found that these TMDs were translocated by the SecYEG channel and an associated protein called YidC.
The Sec and Tat pathways of E. coli therefore cooperate to integrate Sco2149 correctly into the cell membrane. The authors now want to check that Sco2149 is processed similarly in S. coelicolor and to investigate how transport is switched between the Sec and Tat machineries after the Rieske protein’s first two TMDs have been translocated. The researchers suspect that collaboration between the Sec and Tat systems may be more widespread, especially if internal twin-arginine motifs can be found in other bacterial membrane proteins.