Intracellular membranes (green) accumulate when the translocon is disturbed.

Extensive piles of internal membranes accumulate when protein targeting to membranes is interrupted in E. coli, as seen in an article by Herskovits et al. on page 465. The membranes contain ribosomes and a receptor with an early function in ribosome targeting.

The receptor is FtsY, a homologue of the eukaryotic signal recognition particle (SRP) receptor. Recent evidence suggests that, unlike eukaryotes, E. coli do not need SRP for ribosome targeting to the membrane, where membrane proteins will be inserted. However, the process does depend on the receptor FtsY. In the new paper, FtsY function is highlighted.

The result is a visual confirmation that FtsY brings ribosomes to the membrane even in the absence of one of its ligands, the SRP. Herskovits et al. disrupted steps downstream of this event—the transfer of ribosomes to the translocon, which inserts newly translated membrane proteins into the membrane—by depleting cells of SRP or translocon components. Thus, the normally transient association of FtsY with ribosomes was stalled, and novel membrane-bound FtsY–ribosome complexes accumlated in a membrane network. The networks lie close to and might be derived from the cytoplasmic membrane. The fact that SRP is not necessary for ribosome targeting to the membrane begs a new question—how do mRNAs encoding membrane proteins find their way to membrane-bound ribosomes? ▪