Characterisation of SS4 and MRC in starch granule initiation

Chen, Jiawen (2022) Characterisation of SS4 and MRC in starch granule initiation. Doctoral thesis, University of East Anglia.

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Starch is the main storage carbohydrate in plants, important for maintaining plant growth. Starch granules in the leaves of most green plants look similarly disc-shaped, but there is a huge diversity in storage starch granule morphology across species and tissues. Starch granule initiation determines the size, shape and number of granules that form within a plastid, but we do not yet know its mechanism. Several granule initiation proteins have been discovered in Arabidopsis, with Starch Synthase 4 (SS4) at the centre, as the only enzyme. One of the direct biochemical interactors of SS4 is Myosin Resembling Chloroplast protein, MRC.

Here, I explored the biochemistry and function of SS4 and MRC, to better understand their roles during granule initiation in Arabidopsis and wheat. Wheat endosperm contains a bimodal size distribution of small, spherical B-type granules, and large, flattened A-type granules. I characterised a previously unknown role for MRC in repressing the initiation of B-type granules during early wheat endosperm development, resulting in a higher volume percentage of B-type granules in mature mrc endosperm than in wild type. However, wheat mrc leaves had fewer granules per chloroplast than wild type, suggesting tissue-specific protein functions. Additionally, wheat SS4 and MRC can interact with each other along the entire SS4 protein. However, Arabidopsis in planta studies suggest that the SS4-MRC interaction might be transient. Size exclusion chromatography demonstrates that Arabidopsis and wheat SS4 exist at high molecular weight in leaves, and in vitro. Interestingly, negative stain transmission electron microscopy of the recombinant AtSS4 reveals densities that are consistent with an AtSS4 dimer, suggesting that stable AtSS4 dimers could assemble into more transient homooligomeric complexes, or the AtSS4 dimer might have an extended shape. Together, this work advances our knowledge on the biochemical context of SS4 function, with potential for further structural characterisation of SS4.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Biological Sciences
Depositing User: Chris White
Date Deposited: 24 Apr 2023 08:38
Last Modified: 24 Apr 2023 08:38


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