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Post translational modification in plants.
- 1. POST TRANSLATIONAL MODIFICATION IN PLANTS Amit Dhuri 1
- 2. POST TRANSLATIONAL MODIFICATION • Chemical modification of proteins is the addition of small chemical group to the proteins. • Proteins undergoes reversible or irreversible Post Translational Modifications (PTMs) of specific amino acid residues. • Many metabolic enzymes and their regulators undergoes variety of PTMs resulting in changes in oligomeric state, reactivation or deactivation, stabilization or degradation. • PTMs play key role in plants through their impact on signalling, gene expression, protein stability and interaction. 2
- 3. VARIOUS TYPES OF CHEMICAL MODIFICATION PTMS 3 Fig. 1
- 5. 1. PHOSPHORYLATION Phosphorylation is the addition of phosphate group to the protein. It is catalyzed by kinases which transfers a phosphoryl group from ATP to the hydroxyl group of the proteins. Principally on Serine (75%-80%), Threonine (15%-20%) and Tyrosine (1%-5%) residues. Plant genome encodes twice as many kinases compared to mammalian Genome. Phosphatases are responsible for removing phosphorylated residues. 5 Fig. 2
- 6. 6 IMPORTANCE This PTM is seen in the enzymes that are involved in the photorespiratory pathways Site: chloroplasts, peroxisomes and mitochondria These plant protein phosphorylation are required for light and dark reaction of photosynthesis, nitrogen metabolism and secondary metabolism Phosphorylation helps in regulation of various enzymes involved in various photosynthetic pathways.
- 7. 7 Fig. 3
- 8. 2. UBIQUITINATION Protein ubiquitination consists of the covalent binding of a single ubiquitin molecule or chain of ubiquitin molecules to lysine residues of proteins. Monoubiquitylation and Polyubiquitylation Ubiquitin protein binds to the substrate by sequential participation of three enzymes: a) Ubiquitin activating enzyme (E1) b) Ubiquitin conjugating enzyme (E2) c) Ubiquitin Protein ligase (E3) Ubiquitination consists of three steps: 1. Activation 2. Conjugation 3. Ligation 8
- 9. 1. Activation Ubiquitin is activated by two step through E1 ubiquitin activating enzyme. The initial step involves production of ubiquitin-adenylate intermediate. The second step involves ubiquitin transfer to an active site cysteine residue with release of AMP. 2. Conjugation E2 ubiquitin conjugating enzyme catalyze the transfer of ubiquitin from E1 to the active site cysteine of the E2 3. Ligation E3 ubiquitin creates an isopeptide bond between a lysine of the target protein and the C-terminal glycine of ubiquitin. 9 Fig. 4
- 10. • E3 enzymes possess one of the two domains: Homologous to E6-AP C-Terminus (HECT) and Really Interesting New Gene (RING)/ U box domain. • HECT domain E3s: binds to E2 enzyme at N-terminal and ubiquitin through thioester linkage at C-terminal. RING domain E3s are further divided into 2 types: i) single unit containing RING/U-box domain which directly binds to substrate ii) multisubunit containing either RBX1 (Ring box 1)or APC11 (Anaphase Promoting Complex 11) The multi subunit E3 ligases function in many complexes which include SCF (SKP1-CULLIN- F-box), CUL3 (CULLIN 3) BTB/POZ (Bric a brac, Tramtrack and Broad complex/Pox virus and Zinc finger), CUL4-DDB1 (UV-Damaged DNA Binding Protein 1) and APC (Anaphase Promoting Complex) in plants They recognize proteins and ubiquitinate specific substrate to be processed by the proteasomal degradation system 10
- 11. 11 Fig. 5
- 12. IMPORTANCE Ubiquitin mediated proteasomal degradation of protein It helps to control protein content of the cell, including enzymes like kinases and plant hormones Ubiquitin mediated degradation plays an important role in abiotic stress conditions: ABI 5 is regulated by E3 ligases like KEG. Important role in inducing plant immunity: PRR and hypersensitive cell death by MYB30 transcription factor 12
- 13. 3. LIPID PROTEIN MODIFICATION Proteins can be co-translationally or post translationally modified by covalent liquid attachments. Lipidation increases the affinity of proteins to cellular membranes, helps in cellular localization and targeting signals, protein-protein interaction. Four types of lipid protein modification known in plants: 1. N-myristoylation 2. S- Palmitoylation 3. S-Prenylation 4. C- terminal glycosyl phosphatidylinositol (GPI) anchor. 13
- 14. 1. N-Myristoylation i) Attachment of myristoyl group a 14- carbon saturated fatty acid to N-terminal end of protein. ii) Its is facilitated by N-myristoyltransferase (NMT) enzyme and uses myristoyl-coA as the substrate. 2. S-Palmitoylation: i) Addition of (C16) palmitoyl group to cysteine(S) residue from palmitoyl Co-A ii) Palmitoyl acyl transferases (PATs) enzyme favours this step 3. S-Prenylation: i) Addition of farnesyl (C15) or geranylgeranyl (C20) group to proteins. ii) Enzyme involved is farnesyl transferase (FT) or geranylgeranyl transferases (GGT) 4. C- terminal Glycosyl Phosphatidylinositol (GPI) anchor: i) Glycosyl phosphatidylinositol anchors are attached to protein through an amide bond between mannose-6- phosphoethanolamine and the C-terminal carboxyl Group. ii) These GPI anchored proteins are present in the outer leaflet of the plasma membrane LIPID PROTEIN MODIFICATION 14
- 15. 15 Fig.6
- 16. IMPORTANCE C-TERMINAL GLYCOSYL PHOSPHATIDYLINOSITOL (GPI) ANCHOR More than 200 proteins are GPI linked proteins These proteins are involved in cell wall formation, accumulation of cuticular wax, cellular signaling and proteolysis. S-PRENYLATION More than 700 proteins are involved in specific biological processes undergoes prenylation. Prenylated proteins function: transcription, regulation of cell cycle, alteration of cell wall, homeostasis and defence mechanism. S-PALMITOYLATION More than 600 proteins are predicted to be palmitoylated Few palmitoylated proteins: MAT kinases, membrane transporters and ATPases N-MYRISTOYLATION About more than 400 proteins are predicted to be myristoylated Myristoylated proteins: kinases, phosphatases, thioredoxins, GTP- binding proteins, transcription factors. 16
- 17. 4. METHYLATION Addition of Methyl group to the protein usually at lysine and Arginine residues. Methylation are carried out by Methyltransferases where methyl group is transferred on the Nitrogen or oxygen molecule on the proteins. S-adenosylmethionine (SAM) acts as methyl donor, releasing S-adenosyl homocysteine (SAH) Lys methylation can be reversed by various Lys demethylases, but Arg demethylases have yet to be identified. Arg (Monomethylation and demethylation) and Lys (Dimethylation and trimethylation) were observed. 17 Fig. 8
- 18. IMPORTANCE First discovered Lys methylated proteins is Rubisco due to this first systematic Lys and Arg methylation studies focused on chloroplast Arabidopsis methyltransferase (PrmA) is responsible for methylation of ribosomal protein L11 in chloroplast and mitochondria Other plastid methyl transferases and substrates have been identified like Plastid Transcriptionally Active Chromosomes complex14 (PTAC14) Trimethylated Lys residues were observed for an Arabidopsis mitochondrial elongation factor and NAD-malate dehydrogenase. 18
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