A novel precursor recognition element facilitates posttranslational binding to the signal recognition particle in chloroplasts

Signal recognition particles (SRPs) in the cytosols of prokaryotes and euka ryotes are used to target proteins to cytoplasmic membranes and the endopla smic reticulum, respectively. The mechanism of targeting relies on cotransl ational SRP binding to hydrophobic signal sequences. An organellar SRP iden tified in chloroplasts (cpSRP) is unusual in that it functions posttranslat ionally to localize a subset of nuclear-encoded thylakoid proteins. In assa ys that reconstitute thylakoid integration of the light harvesting chloroph yll-binding protein (LHCP), stromal cpSRP binds LHCP posttranslationally to form a cpSRP/LHCP transit complex, which is believed to represent the LHCP form targeted to thylakoids. In this investigation, we have identified an 18-aa sequence motif in LHCP (L18) that, along with a hydrophobic domain, i s required for transit complex formation. Fusion of L18 to the amino termin us of an endoplasmic reticulum-targeted protein, preprolactin, led to trans it complex formation whereas wild-type preprolactin exhibited no ability to form a transit complex. In addition, a synthetic L18 peptide, which compet ed with LHCP for transit complex formation, caused a parallel inhibition of LHCP integration. Translocation of proteins by the thylakoid Sec and Delta pH transport systems was unaffected by the highest concentration of L18 pe ptide examined. Our data indicate that a motif contained in L18 functions i n precursor recruitment to the posttranslational SRP pathway, one of at lea st four different thylakoid sorting pathways used by chloroplasts.