Fig. 1. Selective protein targeting by SRP in translation lysate.
a Projections of the EM densities of structurally resolved RPBs onto the surface of the 80S ribosome (PDB: 4UG0; light gray)78 viewed from the tunnel exit (marked by an asterisk). The electron density map of the RPBs (SRP: EMD-30376, green; NAC: EMD-493823, magenta; RAC: EMD-610579, red; and NatA/E: EMD-020280, blue) were overlayed using the structure of 80S in Chimera (UCSF). The densities of ribosomal proteins surrounding the tunnel exit are shown in different shades of gray, with dotted areas indicating the parts buried underneath the ribosomal surface. Due to the limited resolutions of the original EM maps, the projections of NAC and RAC are likely incomplete. b Composition of the signal sequence variants tested in this work. c Translocation assay showing the specific targeting of pPL(wt) and pPL(ss). SRP-dependent cotranslational protein translocation were measured for the indicated nascent proteins as described in the Methods section and analyzed by SDS-PAGE and autoradiography. The bands for preprolactin (pPL) and signal sequence-cleaved prolactin (PL) are indicated. d Quantification of the data in c. The values for pPL(ssmt) and pPL(ssmt2) are very close and overlap with each other in the figure. Translocation efficiency was calculated using Eq. (3) in the Methods section. Source data for c and d are provided in the Source Data file.