Elucidating Mechanisms that Direct Chloroplast Encoded Proteins to the Thylakoid Membrane

Abstract

The thylakoid membrane is the site of the light reactions of photosynthesis and is the destination for many integral membrane proteins that are involved in photosynthesis. My research aims to elucidate the mechanisms that target chloroplast-encoded proteins to the thylakoid membrane. Previous studies have revealed three separate pathways by which nuclear-encoded proteins are targeted to the thylakoid membrane, each of which evolved from secretion systems in the cyanobacterial ancestor. Plastid-encoded proteins are hypothesized to use these pathways to integrate into the thylakoid membrane as well. By profiling ribosomes in separated soluble and membrane fractions, the Barkan lab showed that many plastid mRNAs are translated on ribosomes that are anchored to the thylakoid membrane, and that the proteins encoded by these mRNAs are inserted into the thylakoid co-translationally. I conducted partitioned ribosome profiling experiments on maize mutants that lack each of the three thylakoid membrane translocon machineries to reveal which thylakoid targeted proteins are excluded from co-translational membrane integration in these specific mutants. I found that the loss of certain translocon machinery had differential effects on the cotranslational targeting of plastid-encoded thylakoid membrane proteins.