Chloroplast SRP54 interacts with a specific subset of thylakoid precursor proteins.
Signal recognition particles (SRPs) have been identified in organisms as diverse as mycoplasma and mammals; in several cases these SRPs have been shown to play a key role in protein targeting. In each case the recognition of appropriate targeting signals is mediated by SRP subunits related to the 54-kDa protein of mammalian SRP (SRP54). In this study we have characterized the specificity of 54CP, a chloroplast homologue of SRP54 which is located in the chloroplast stroma. We have used a nascent chain cross-linking approach to detect the interactions of 54CP with heterologous endoplasmic reticulum-targeting signals. 54CP functions as a bona fide signal recognition factor which can discriminate between functional and non-functional targeting signals. Using a range of authentic thylakoid precursor proteins we found that 54CP discriminates between thylakoid-targeting signals, interacting with only a subset of protein precursors. Thus, the light-harvesting chlorophyll a/b-binding protein, cytochrome f, and the Rieske FeS protein all showed strong cross-linking products with 54CP. In contrast, no cross-linking to the 23- and 33-kDa proteins of the oxygen-evolving complex were detected. The selectivity of 54CP correlates with the hydrophobicity of the thylakoid-targeting signal and, in the case of light-harvesting chlorophyll a/b-binding protein, with previously determined transport/integration requirements. We propose that 54CP mediates the targeting of a specific subset of precursors to the thylakoid membrane, i.e. those with particularly hydrophobic signal sequences.
High
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S
School of Biological Sciences, University of Manchester, 2.205 Stopford Building Oxford Road, Manchester M13 9PT, United Kingdom. SHIGH@fs2.scg.man.ac.uk
Henry
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R
Mould
R M
RM
Valent
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Q
Meacock
S
S
Cline
K
K
Gray
J C
JC
Luirink
J
J
D000595:Amino Acid Sequence; D001692:Biological Transport; D002451:Cell Compartmentation; D002736:Chloroplasts / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism; D045342:Light-Harvesting Protein Complexes; D008565:Membrane Proteins / Q000378:metabolism*; D008969:Molecular Sequence Data; D018532:Pisum sativum; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism; D010940:Plant Proteins / Q000378:metabolism*; D011485:Protein Binding; D011498:Protein Precursors / Q000378:metabolism*; D021382:Protein Sorting Signals / Q000378:metabolism; D018271:Signal Recognition Particle / Q000378:metabolism*
D045342:Light-Harvesting Protein Complexes; D008565:Membrane Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D010940:Plant Proteins; D011498:Protein Precursors; D021382:Protein Sorting Signals; D018271:Signal Recognition Particle