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. 2019 Jan 15;47(6):3184–3196. doi: 10.1093/nar/gky1324

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© The Author(s) 2019. Published by Oxford University Press on behalf of Nucleic Acids Research.

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com

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Figure 6. — Scheme of SRP/SR/ribosome interactions in the SRP cycle. Left panel: SRP(S) strongly binds to the ribosome in the scanning state (individual contributions in nM determined by MST are discerned). SRP54 reveals a dual binding mode by its M (high affinity (1)) and NG domains (low affinity (2)). Contributions of the Alu domain and SRβ are not defined. SRP68/72 interactions (4) are ultrasensitive, include more than the C4-contact (3) established by SRP72–RBD, and involve the C-terminus (C). Middle panel: in the pre-handover state (engaged) interactions are reinforced mainly by signal sequence recognition of SRP54M (1′) and SRP54NG is locked in place (12,14). Upon SR binding, ribosome affinity is as strong although the TC complex of the NG domains is known to dissociate from the ribosome (20). Right panel: Upon translocon docking, the TC relocates and the signal is handed-over. No structure of any post-handover complex is available and individual contributions are elusive. TC re-localization induces a rotation of SRPS in respect to the ribosome and a modification of the C4-contact (3′) at the distal site (20,44). Only upon GTP-hydrolysis, SRP dissociates from the ribosome (indicated by asterisks) and the SRP cycle is completed. ^#: MST data for SRPS/SR indicate a very tight RNC interaction before GTP-hydrolysis occurs (although the Alu domain and translocon were missing in the assay).