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 subuni ts related to the 54-kDa protein of mammalian SRP (SRP54), In this stu dy we have characterized the specificity of 54CP, a chloroplast homolo gue of SRP54 which is located in the chloroplast stroma. We have used a nascent chain cross-linking approach to detect the interactions of 5 4CP with heterologous endoplasmic reticulum-targeting signals. 54CP fu nctions as a bona fide signal recognition factor which can discriminat e between functional and non-functional targeting signals, Using a ran ge of authentic thylakoid precursor proteins we found that 54CP discri minates between thylakoid-targeting signals, interacting with only a s ubset 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-linkin g 60 the 23- and 33-kDa proteins of the oxygen-evolving complex were d etected, The selectivity of 54CP correlates with the hydrophobicity of the thylakoid-targeting signal and, in the case of light-harvesting c hlorophyll a/b-binding protein, with previously determined transport/i ntegration requirements, We propose that 54CP mediates the targeting o f a specific subset of precursors to the thylakoid membrane, i.e. thos e with particularly hydrophobic signal sequences.