Date of Graduation

12-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Cell & Molecular Biology (PhD)

Degree Level

Graduate

Department

Biological Sciences

Advisor

Suresh Thallapuranam

Committee Member

Colin Heyes

Second Committee Member

David McNabb

Third Committee Member

Ralph Henry

Keywords

chloroplast, cpSRP, generic chaperone, protein targeting, signal recognition particle

Abstract

Protein targeting is a vital cellular function. The signal recognition particle (SRP) pathway is a universally conserved targeting system present in the cytosol and used to co-translationally target many proteins to the inner membrane of prokaryotes and the endoplasmic reticulum of eukaryotes. The chloroplast has a homologous SRP system which post-translationally targets light harvesting chlorophyll binding proteins (LHCPs) to the thylakoid membrane for integration. The chloroplast SRP (cpSRP) is a heterodimer with a 54 kDa subunit equivalent to SRP54 in the canonical pathway. In addition, cpSRP contains a novel 43 kDa subunit which is a unique and irreplaceable component. cpSRP43 is central to targeting the highly hydrophobic LHCPs to the Albino3 translocase at the thylakoid membrane by operating as a chaperone capable even of disaggregating LHCPs without any external energy input. cpSRP43 has multiple binding partners in the cpSRP pathway. Many details about these binding interactions have been discovered however exact residual details regarding these interactions still requires elucidation. A structure for cpSRP43 bound to cpSRP54 was determined by X-Ray crystallography although cpSRP43 also functions in free form in the chloroplast. The results of this study demonstrate the significant amount of structural flexibility and thermal stability cpSRP43 as well as its potential use as a generic chaperone for proteins outside of the chloroplast. The structure of the c-terminal end of Albino3 (cAlb) was also investigated here since the exact binding interface between cpSRP43 and cAlb is still under debate. The results here reveal a region in cAlb which has a high propensity toward structure. This region may prove to be important in binding to cpSRP43 and could lead toward a better understanding of the process of integration for LHCP.

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