Characterization of Spiroplasma citri Fibril:Insights into polymerization interface and interaction with other cytoskeletal proteins

Abstract

Spiroplasma is a cell-wall less helical bacterium. Electron microscopy studies show the presence of a flat, monolayered and membrane-bound ribbon-like cytoskeleton structure which consists of Fibril and 5 MreB paralogs. In the absence of any outer appendages, the cytoskeletal ribbon is thought to be responsible for maintaining the cell shape and motility of this organism. My Ph.D. work aims to characterize Fibril and MreBs using structural and biochemical approaches to understand the mechanism by which these proteins contribute to cell shape and motility. For characterization of Fibril, based on the sequence homology of the N-terminal half of the protein to 5’- methylthioadenosine nucleosidase (MTAN), nucleosidase activity was checked for Fibril. We observed that in spite of similar domain architecture, Fibril does not exhibit any nucleosidase activity. Fibril protein forms constitutive filaments. In absence of any information regarding the filament interface, we incorporated EGFP as a fluorescent tag at the loop regions based on the sequence based secondary structure prediction. We found that despite the EGFP insertions at multiple positions, Fibril readily forms filaments and might not be stable as a monomer. Earlier studies have shown that MreB5 interacts with lipids and Fibril. Our results show that Fibril and MreB5 interact in a nucleotide dependent manner similar to MreB5- liposome interaction. Our findings reveal that Fibril does not possess any enzymatic activity or ligand-dependent polymerization and that it acts as a scaffold for cell shape maintenance which is regulated by MreB5 filament dynamics in Spiroplasma. For biochemical characterization like polymerization dynamics, ATP hydrolysis and checking interaction with other MreBs and Fibril, purification of MreB1 was initiated. Purification trials of MreB1 are also discussed in the thesis.