Dynamic Regulations of Co-translational Protein Targeting by the Signal Recognition Particle Receptor in E. coli and Human

Abstract

The Signal Recognition Particle (SRP) and its receptor (SR) co-translationally deliver the majority of secretory and membrane proteins to their membrane destinations. SRP recognizes and binds to the cargo, and SR recruits the transit complex to the target membrane. Precise timing and coordination are encoded into the interactions between SRP and SR in response to the cargo and environment to enable efficient and accurate targeting. This dissertation explores the regulation mechanisms of SRP membrane recruitment by SR in both bacteria and eukaryotes. The bacterial SR is a peripheral membrane protein and recruits SRP by adjusting its membrane-binding modes. A complete kinetic model of SRP membrane recruitment by the SR was established based on direct observations and quantitative analyses of protein-membrane binding events at single-molecule resolution. Biochemical and cell biological examinations validated the physiological significance of the newly discovered dynamic membrane-binding mode of the SR. The two-stage SR membrane binding mechanism ensures both speed and specificity in bacterial co-translational protein targeting. In eukaryotes, a proper initial recognition of SRP sets the right course for membrane delivery of the transit complex, as eukaryotic SR is anchored on the endoplasmic reticulum. A Molecular Recognition Feature (MoRF) in the disordered linker domain of eukaryotic SR was identified and characterized. The SR MoRF element enables rapid SRP-SR assembly in response to the ribosomal content of the cargo. The stimulation by SR MoRF is only present in eukaryotes and is likely a functional replacement of the tetraloop in bacterial SRP RNA whose stimulatory role was abolished during evolution.