A unique sequence motif in the 54-kDa subunit of the chloroplast signal recognition particle mediates binding to the 43-kDa subunit.
Chloroplasts contain a novel type of signal recognition particle (cpSRP) that consists of two proteins, cpSRP54 and cpSRP43. cpSRP is involved in the post-translational targeting of the nuclear encoded light-harvesting chlorophyll-binding proteins (LHCPs) to the thylakoid membrane by forming a soluble cpSRP.LHCP transit complex in the stroma. Despite high sequence homology between chloroplast and cytosolic SRP54 proteins, the 54-kDa subunit of cpSRP is unique in its ability to bind cpSRP43. In this report, we identified a 10-amino acid long segment of cpSRP54 that forms the cpSRP43-binding site. This segment is located at position 530-539 close to the C terminus of cpSRP54. In addition, we demonstrate that arginine at position 537 is essential for binding cpSRP43 and that mutation of arginine 536 drastically reduced cpSRP43 binding. Mutations within the cpSRP43-binding site of cpSRP54 that reduced or completely abolished cpSRP complex formation also did inhibit transit complex formation and integration of LHCP into the thylakoid membrane, reflecting the importance of these residues for LHCP targeting. Alignment studies revealed that the cpSRP43-binding site is conserved in chloroplast SRP54 proteins and is not present in any SRP54 subunit of cytosolic SRPs.
Funke
Silke
S
Lehrstuhl für Allgemeine und Molekulare Botanik, Ruhr-Universität Bochum, 44780 Bochum, Germany.
Knechten
Thomas
T
Ollesch
Julian
J
Schünemann
Danja
D
D000595:Amino Acid Sequence; D001665:Binding Sites; D002736:Chloroplasts / Q000378:metabolism*; D003001:Cloning, Molecular; D017931:DNA Primers; D008969:Molecular Sequence Data; D008970:Molecular Weight; D016297:Mutagenesis, Site-Directed; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D021122:Protein Subunits / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics; D016415:Sequence Alignment; D017384:Sequence Deletion; D017386:Sequence Homology, Amino Acid; D018271:Signal Recognition Particle / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
D017931:DNA Primers; D010940:Plant Proteins; D021122:Protein Subunits; D011994:Recombinant Proteins; D018271:Signal Recognition Particle