Co-translational protein targeting by the SRP pathway

Signal Recognition Particle (SRP) is a universally conserved protein targeting machine that mediates the delivery of roughly 30% of the newly-synthesized proteome to the eukaryotic endoplasmic reticulum or the bacterial plasma membrane. As such, it has served as a paradigm to understand the molecular basis of protein localization in all cells. Our work on the bacterial SRP has established a quantitative framework that illustrates how a series of conformational changes in the SRP and SRP receptor enable this pathway to accurately select its substrates and efficiently deliver them to the target membrane (Click to see the movie). Compared to the bacterial SRP, the mammalian SRP contains five additional protein subunits whose precise roles and mechanism of action remain elusive. Understanding the mammalian SRP will likely unravel new layers of control and regulation in higher eukaryotic cells.

Conformational Changes in the SRP–SR GTPase Dimer Regulate Co-Translational Protein Targeting.

SRP54 and SR are in blue and green, respectively. T denotes GTP; D denotes GDP. (A) Cargo, membrane, and translocon drive multiple rearrangements during SRP–SR dimerization, as described in the text. The perpendicular symbol (?) denotes the effect of RNC in delaying GTPase activation. Top panel: the crystal structures of free SRP54 (left; 1QZW) and the SR NG domain (right; 2Q9C). The SRP54 NG and M domains are in light and dark blue, respectively, the linker between the two domains is in gray, and the SRP RNA is in tan. The bound GTP analogs are in ‘spacefill’. Right panel: cryo-EM model of the ‘early’ SRP–SR complex bound to RNC (2XKV); the RNC is not shown for clarity. Bottom panel: the crystal structure of the ‘closed’ SRP–SR NG-domain complex (1RJ9). Left panel: the crystal structure of the ‘activated’ complex (2XXA). (B) GTPase rearrangements in SRP and SR drive distinct molecular steps during targeting. The steps are numbered to be consistent with (A). Step 1, RNC with a signal sequence (magenta) binds SRP. Step 2, RNC-loaded SRP forms a stabilized ‘early’ targeting complex with SR. Step 3, phospholipids drive rearrangement to the ‘closed’ state. Step 4, SecYEG drives rearrangement to the ‘activated’ state, which frees the ribosome for subsequent unloading. Step 5, the RNC is unloaded from SRP onto the SecYEG complex, and GTP hydrolysis drives the disassembly and recycling of SRP and SR. RNC, ribosome–nascent chain complex; SR, signal recognition particle receptor; SRP, signal recognition particle.

Selected Publications:

  • Zhang, X., Schaffitzel, C., Ban, N., and Shan, S. (2009) Proc. Natl. Acad. Sci. 106, 1754-1759. "Multiple conformational switches in a GTPase complex control co-translational protein targeting." PMID: 19174514.
  • Zhang, X., Rashid, R., Wang, K., and Shan, S. (2010) Science 328, 757-760. "Sequential checkpoints govern substrate selection during cotranslational protein targeting.” PMID: 20448185.
  • Shen, K., Arslan, S., Akopian, D., Ha, T., and Shan, S. (2012) Nature 492 (7428):271-5. “Activated GTPase movement on an RNA scaffold drives cotranslational protein targeting.” PMID: 23235881.
  • Saraogi, I., Akopian, D., and Shan, S. (2014) J. Cell Biol. 205, 693-706. “Regulation of cargo recognition, commitment, and unloading drives cotranslational protein targeting.” PMID: 24914238.
  • Zhang, X. and Shan, S. (2014) Annu. Rev. Biophys. 43, 381-408. “Fidelity of cotranslational protein targeting by the Signal Recognition Particle.” PMID: 24895856.