Cotranslational signal independent SRP preloading during membrane targeting

Abstract

Ribosome-associated factors must faithfully decode the limited information available in nascent polypeptides to direct them to their correct cellular fate ¹ . It is unclear how the low complexity information exposed by the nascent chain suffices for accurate recognition by the many factors competing for the limited surface near the ribosomal exit site ^{2, 3} . Questions remain even for the well-studied cotranslational targeting cycle to the endoplasmic reticulum (ER), involving recognition of linear hydrophobic Signal Sequences (SS) or Transmembrane Domains (TMD) by the Signal Recognition Particle (SRP) ^{4, 5} . Intriguingly, SRP is in low abundance relative to the large number of ribosome nascent chain complexes (RNCs), yet it accurately selects those destined to the ER ⁶ . Despite their overlapping specificities, SRP and the cotranslational Hsp70 SSB display exquisite mutually exclusive selectivity in vivo for their cognate RNCs ^{7, 8} . To understand cotranslational nascent chain recognition in vivo, we interrogated the cotranslational membrane targeting cycle using ribosome profiling (herein Ribo-seq) ⁹ coupled with biochemical fractionation of ribosome populations. Unexpectedly, SRP preferentially binds secretory RNCs before targeting signals are translated. We show non-coding mRNA elements can promote this signal-independent SRP pre-recruitment. Our study defines the complex kinetic interplay between elongation and determinants in the polypeptide and mRNA modulating SRP-substrate selection and membrane targeting.