Temporal Regulation of Signal Recognition Particle During Translation.

Signal recognition particle (SRP) is a universally conserved protein targeting machine that directs newly synthesized proteins to the endoplasmic reticulum (ER). SRP recognizes signal sequences on nascent ER proteins as they emerge from the ribosome and, in response, activates interaction with the SRP receptor (SR) at the ER membrane. Early work suggested that SRP loses targeting competence as the nascent chain elongates; however, the underlying molecular mechanism remains unclear. Here we address this question using a combination of steady-state and single-molecule fluorescence spectroscopy measurements. A Förster resonance energy transfer (FRET) assay revealed increased dynamic excursions of the signal sequence from SRP on ribosomes bearing longer nascent chains, leading to a suboptimal conformation of SRP and its impaired interaction kinetics with SR. In addition, the nascent polypeptide associated complex (NAC) amplifies the effects of longer nascent chains to further exclude SRP from ER targeting. Our findings reveal the profound effects of an elongating nascent polypeptide on the conformation and activity of SRP and a key role of NAC in the temporal regulation of SRP, which together impose a limited window for cotranslational ER protein targeting during protein synthesis.