Date of Graduation

5-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Cell & Molecular Biology (PhD)

Degree Level

Graduate

Department

Cell & Molecular Biology

Advisor/Mentor

Henry, Ralph L.

Committee Member

Goforth, Robyn L.

Second Committee Member

Thallapuranam, Suresh

Third Committee Member

McNabb, David S.

Fourth Committee Member

Ivey, D. Mack

Keywords

Biological sciences; Chloroplasts; Particle; Protein; Signal recognition; Targeting/Insertion; Thylakoid

Abstract

Protein targeting is a fundamental cellular process that directs proteins from their site of synthesis to the site where they function. The signal recognition particle (SRP) dependent targeting pathway is conserved in both eukaryotes and prokaryotes where it co-translationally targets polypeptide chains emerging from ribosomes to the endoplasmic reticulum (eukaryotes) or cytoplasmic membrane (prokaryotes). A structurally unique form of SRP is found in chloroplasts where it functions to post-translationally bind and target a subset of integral thylakoid membrane proteins, the light harvesting chlorophyll binding proteins (LHCPs). Mature LHCPs bind chlorophyll a/b and function in photosynthetic light capture. Like many other chloroplast proteins, LHCPs are nuclear encoded and synthesized in the cytosol. Following their import into the chloroplast stroma, LHCPs associate with chloroplast SRP (cpSRP), which maintains LHCP solubility and initiates targeting of LHCP to the thylakoid membrane via an cpSRP receptor (cpFtsY) at the thylakoid membrane. Both cpSRP and cpFtsY are GTPases and associate at the thylakoid by a mechanism that requires GTP binding by both proteins. Subsequent insertion of LHCP into the lipid bilayer is mediated by a protein insertase Albino3 (Alb-3), which binds cpSRP to stimulate LHCP release from cpSRP and GTP hydrolysis by both cpSRP and its receptor. Work here has focused on studies to understand mechanistic details of the cpSRP targeting pathway and better understand the timing of targeting events at the membrane. The results provide support for a structure-based chronology of protein interactions between LHCP targeting substrates, cpSRP, cpFtsY, and Alb-3. They also demonstrate that GTP hydrolysis by cpSRP and its receptor at the membrane is not necessary for LHCP insertion by Alb-3, but serves to maintain an available pool of Alb-3 insertase at the membrane by stimulating the exit of cpSRP targeting components following release of LHCP from cpSRP to Alb-3.

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