Signal-sequence recognition by an Escherichia coli ribonucleoprotein complex

HYDROPHOBIC signal-sequences direct the transfer of secretory proteins across the inner membrane of prokaryotes and the endoplasmic reticulum membranes of eukaryotes1. In mammalian cells, signal-sequences are recognized by the 54K protein (Mr 54,000) of the signal recognition particle (SRP)2,3 which is believed to hold the nascent chain in a translocation-competent conformation until it contacts the endoplasmic reticulum membrane4. The SRP consists of a 7S RNA and six different polypeptides. The 7S RNA and the 54K signal-sequence-binding protein (SRP54) of mammalian SRP exhibit strong sequence similarity to the 4.5S RNA and P48 protein (Ffh) of Escherichia coli5-7 which form a ribonucleoprotein particle. Depletion of 4.5S RNA or overproduction of P48 causes the accumulation of the β-lactamase precursor, although not of other secretory proteins8,9. Whether 4.5S RNA and P48 are part of an SRP-like complex with a role in protein export is controversial. Here we show that the P48/4.5S RNA ribonucleoprotein complex interacts specifically with the signal sequence of a nascent secretory protein and therefore is a signal recognition particle.