Function of a chloroplast SRP in thylakoid protein export

Protein export systems derived from prokaryotes are used to transport proteins into or across the endoplasmic reticulum, the mitochondrial inner membrane, and the chloroplast thylakoid membrane. Signal recognition particle (SRP) and its receptor are essential components used exclusively for cotranslational export of endomembrane and secretory proteins to the endoplasmic reticulum in eukaryotes and export of polytopic membrane proteins to the cytoplasmic membrane in prokaryotes. An organellar SRP in chloroplasts (cpSRP) participates in cotranslational targeting of chloroplast synthesized integral thylakoid proteins. Remarkably, cpSRP is also used to posttranslationally localize a subset of nuclear encoded thylakoid proteins. Recent work has begun to reveal the basis for cpSRP's unique ability to function in co- and posttranslational protein localization, yet much is left to question. This review will attempt to highlight these advances and will also focus on the role of other soluble and membrane components that are part of this novel organellar SRP targeting pathway.