Plastids Retained a Functional Prokaryotic-Like Protein Secretion Pathway That Can Export Proteins Synthesized in Chloroplasts into the Cytoplasm

Abstract Background Chloroplasts are semiautonomous organelles considered to have evolved from prokaryotic organisms like Cyanobacterium through endosymbiosis. During the course of evolution spanning over a billion years, a part of the organellar genome has migrated to the nucleus, and proteins encoded by such genes are synthesized in the cytoplasm and then imported into chloroplasts for their functions. To import nuclear-encoded and cytoplasm-synthesized proteins, chloroplasts have evolved multiple pathways, and some of them resemble prokaryotic protein export pathways operating in opposite directions. However, it is unknown whether any prokaryotic protein export mechanisms are functionally conserved in present-day land plant chloroplasts, allowing proteins synthesized in the chloroplasts to be exported into the cytoplasm. Results To understand the existence of any functional protein export pathway in chloroplasts, the coding region of a bacterial signal peptide from Bacillus subtilis, involved in the export of a cellulolytic enzyme (BSX), was translationally fused in-frame with GFP at 5’ end to create SP:GFP fusion protein and expressed in chloroplasts. Here we present data providing evidence that shows the existence of a functional protein export mechanism in the chloroplasts, similar to the prokaryotic protein secretion mechanism, which can be exploited for crop improvement and other biotechnological applications. Conclusions Evidence for protein export and prior knowledge about protein import by chloroplasts indicate the presence of unique bidirectional trafficking across the chloroplast envelope. Nonetheless, additional studies are needed to fully understand the molecular components and exact mechanisms underlying protein secretion by chloroplasts.