The nascent polypeptide-associated complex modulates interactions between the signal recognition particle and the ribosome

BACKGROUND The first step in the co-translational targeting of secretory proteins to the endoplasmic reticulum membrane involves the recognition of signal sequences by the 54 kDa subunit of the signal recognition particle (SRP) as they emerge from the ribosome. It has recently been proposed that the nascent polypeptide-associated complex (NAC) contributes to the fidelity of targeting by modulating interactions that occur between the ribosome-nascent chain complex, the SRP and the endoplasmic reticulum membrane. Precisely how NAC influences SRP function is presently unclear. RESULTS We have used immunoblotting experiments to monitor interactions between the SRP and the ribosome-nascent chain complex, in the absence and presence of NAC. In the absence of NAC, SRP binds in a high-salt-resistant manner only to ribosomes that contain a signal sequence, confirming the specificity of SRP for signal sequences. Binding of SRP to signalless ribosome nascent chains is observed at lower salt concentrations; however, the amount of SRP bound to this complex is indistinguishable from that bound to ribosomes lacking nascent chains. Thus, this salt-sensitive binding is likely to be the result of interactions between SRP and the ribosome that occur independently of the nascent chain. A minimal particle consisting of SRP54 and SRP RNA is sufficient to confer salt-resistant binding to ribosomes that contain signal sequences, whereas all of the SRP subunits are required for salt-sensitive binding to ribosomes that lack nascent chains. This salt-sensitive binding by SRP is inhibited by the addition of purified NAC. CONCLUSIONS Based on our results, we define two distinct modes of interaction between SRP and the ribosome-nascent chain complex: salt-resistant interactions between SRP54 and signal sequences, and salt-sensitive interactions between additional components of SRP and the ribosome. We conclude that NAC does not directly influence signal sequence recognition by SRP but, rather, that it negatively modulates interactions that occur between SRP and the ribosome itself. These results are discussed in terms of a model wherein SRP and NAC regulate each others' activity during protein targeting.