title: Co-evolution of Two GTPases Enables Efficient Protein Targeting in an RNA-less Chloroplast Signal Recognition Particle Pathway.
abstract: The signal recognition particle (SRP) is an essential ribonucleoprotein particle that mediates the co-translational targeting of newly synthesized proteins to cellular membranes. The SRP RNA is a universally conserved component of SRP that mediates key interactions between two GTPases in SRP and its receptor, thus enabling rapid delivery of cargo to the target membrane. Notably, this essential RNA is bypassed in the chloroplast (cp) SRP of green plants. Previously, we showed that the cpSRP and cpSRP receptor GTPases (cpSRP54 and cpFtsY, respectively) interact efficiently by themselves without the SRP RNA. Here, we explore the molecular mechanism by which this is accomplished. Fluorescence analyses showed that, in the absence of SRP RNA, the M-domain of cpSRP54 both accelerates and stabilizes complex assembly between cpSRP54 and cpFtsY. Cross-linking coupled with mass spectrometry and mutational analyses identified a new interaction between complementarily charged residues on the cpFtsY G-domain and the vicinity of the cpSRP54 M-domain. These residues are specifically conserved in plastids, and their evolution coincides with the loss of SRP RNA in green plants. These results provide an example of how proteins replace the functions of RNA during evolution.
authors.affiliation: From the Division of Chemistry and Chemical Engineering and.
authors.forename: Sowmya
authors.initials: S
authors.lastname: Chandrasekar
authors.affiliation: the Proteome Exploration Laboratory, Division of Biology and Biological Engineering, Beckman Institute, California Institute of Technology, Pasadena, California 91125.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Sweredoski
authors.affiliation: From the Division of Chemistry and Chemical Engineering and.
authors.forename: Chang Ho
authors.initials: CH
authors.lastname: Sohn
authors.affiliation: the Proteome Exploration Laboratory, Division of Biology and Biological Engineering, Beckman Institute, California Institute of Technology, Pasadena, California 91125.
authors.forename: Sonja
authors.initials: S
authors.lastname: Hess
authors.affiliation: From the Division of Chemistry and Chemical Engineering and sshan@caltech.edu.
authors.forename: Shu-Ou
authors.initials: SO
authors.lastname: Shan
mesh_terms: D000595:Amino Acid Sequence; D060365:Chloroplast Proteins / Q000737:chemistry / Q000378:metabolism*; D002736:Chloroplasts / Q000378:metabolism*; D018360:Crystallography, X-Ray; D019143:Evolution, Molecular; D020558:GTP Phosphohydrolases / Q000737:chemistry / Q000378:metabolism*; D010802:Phylogeny; D018087:Plastids / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D021381:Protein Transport; D017386:Sequence Homology, Amino Acid; D018271:Signal Recognition Particle / Q000737:chemistry / Q000378:metabolism*
chemical_list: D060365:Chloroplast Proteins; D018271:Signal Recognition Particle; D020558:GTP Phosphohydrolases
keywords: GTPase; GTPases; chloroplast; fluorescence; protein cross-linking; protein evolution; protein targeting; signal recognition particle (SRP)