PTM: Ribosylation, Acetylation & Methylation
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Post-translational modifications (PTMs) refer to the chemical modifications that occur on proteins after they have been synthesized, specifically during or after the process of translation. These modifications play a crucial role in regulating protein function, localization, stability, and interactions within the cell. they add a layer of complexity and diversity to the proteome, expanding the repertoire of protein functions and regulatory mechanisms in cellular processes.
Acetylation is a common PTM where an acetyl group (-COCH3) is added to specific amino acid residues, typically lysine. This modification is catalyzed by enzymes called acetyltransferases and removed by deacetylases. Acetylation can influence protein-protein interactions, gene expression, enzymatic activity, and protein stability.
Methylation is the addition of a methyl group (-CH3) to specific amino acids, such as lysine, arginine, and histidine. This modification is typically carried out by methyltransferases and can affect protein-protein interactions, gene expression, and chromatin structure. Methylation can have both activating and repressive effects on protein function, depending on the context and the specific amino acid residue being methylated.
Ribosylation involves the addition of a ribose or ribose-phosphate group to specific amino acid residues, such as arginine or cysteine. This modification is catalyzed by enzymes known as ribosyltransferases. Ribosylation can influence protein localization, stability, enzymatic activity, and protein-protein interactions. ADP-ribosylation is a notable example of ribosylation, where ADP-ribose moieties are added to proteins, often involved in signaling and DNA repair processes.
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MDH can undergo post-translational modifications, including acetylation and methylation, which can impact its activity and function.
Acetylation of MDH has been reported in several organisms. For example, in the bacterium Bacillus subtilis, the malate dehydrogenase enzyme undergoes acetylation on specific lysine residues. This acetylation was found to regulate the activity and stability of the enzyme, potentially influencing its involvement in cellular metabolism.
A lysine-to-glutamine (K-to-Q) mutation can mimic the effects of acetylation on proteins. This mutation involves substituting the lysine residue with a glutamine residue, which is structurally similar to acetyl-lysine. Acetylation of lysine residues typically neutralizes the positive charge on the lysine side chain and introduces a bulkier acetyl group. This modification can impact protein structure, interactions, and function. By introducing a glutamine residue, which is also uncharged but has a similar size and shape to an acetyl group, the K-to-Q mutation can mimic the effects of acetylation on the protein.
The K-to-Q mutation is often used as a tool in experimental studies to investigate the functional consequences of acetylation. By comparing the wild-type protein, the acetylated protein, and the protein with the K-to-Q mutation, researchers can gain insights into the potential functional effects of lysine acetylation on protein structure, activity, and interactions.
Methylation of MDH has also been observed in certain organisms. For instance, in the fungus Aspergillus nidulans, malate dehydrogenase can undergo methylation on specific lysine residues. This methylation was found to affect the enzymatic activity and stability of the protein, potentially influencing its role in cellular metabolic processes.
Mimicking protein methylation through mutations is more complex compared to acetylation, as the structural and functional implications of methylation are diverse and context-dependent. However, there are certain types of mutations that can be used to mimic protein methylation to some extent.
Lysine-to-arginine (K-to-R) mutation: Methylation of lysine residues often neutralizes the positive charge on the lysine side chain, and arginine has a positively charged guanidinium group similar to lysine. Therefore, substituting lysine with arginine through a K-to-R mutation can mimic the charge characteristics of methylated lysine, although it does not replicate the exact methylation state.
Alanine or glutamine substitutions: In some cases, alanine (A) or glutamine (Q) substitutions can be used to mimic the absence of methylation. For example, if a lysine residue is methylated and the mutation aims to mimic the unmethylated state, replacing the methylated lysine with alanine or glutamine can eliminate the bulky methyl group and provide a closer approximation to the unmethylated state.
Lysine-to-methionine (K-to-M) mutation: Methionine contains a sulfur atom that can have similar properties to a methyl group. Therefore, in some instances, a K-to-M mutation can be used to mimic the methylation of lysine residues. However, it is important to note that this substitution may not perfectly replicate the structural and functional effects of lysine methylation.
It is crucial to consider that these mutations may only partially mimic the effects of methylation, as the functional consequences of methylation can vary depending on the specific methylated residue, the degree of methylation, and the surrounding protein context. Experimentally studying the effects of these mutations and comparing them with the actual methylation state of the protein is necessary to gain a comprehensive understanding of the functional impact of methylation.
Ribosylation: PARP (Poly(ADP-ribose) polymerase) is an enzyme involved in a process known as ADP-ribosylation. ADP-ribosylation is a post-translational modification that involves the transfer of ADP-ribose moieties from nicotinamide adenine dinucleotide (NAD+) to target proteins. PARP enzymes catalyze this reaction by utilizing NAD+ as a substrate and transferring ADP-ribose units to specific amino acid residues on target proteins, primarily serine, glutamate, and aspartate. The ADP-ribosylation process mediated by PARP enzymes plays a crucial role in various cellular processes, including DNA repair, genomic stability, transcriptional regulation, and signaling pathways. It serves as a regulatory mechanism for protein function and protein-protein interactions. ADP-ribosylation can have diverse functional effects on target proteins. It can alter protein activity, protein-protein interactions, protein localization, and protein stability. The addition of ADP-ribose moieties can introduce a negatively charged group, which can affect protein conformation, function, and protein-protein interactions. ADP-ribosylation is a reversible process, and the removal of ADP-ribose groups is carried out by enzymes known as ADP-ribosyl hydrolases or poly(ADP-ribose) glycohydrolases (PARGs).
Information & Resources
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Relevant Publications
Studying the Lysine Acetylation of Malate Dehydrogenase (JMB 2017)
Acetylation of Malate Dehydrogenase 1 Promotes Adipogenic Differentiation via Activating its Enzymatic Activity (JLR 2012)
Arginine Methylation of MDH1 by CARM1 Inhibits Glutamine Metabolism and Suppresses Pancreatic Cancer (Molecular Cell 2016)
List of Skills/ Techniques Needed: list with link to protocol pages
A list of adopters: Delphine Quinete… (email josephprovost@sandiego.edu to add your name!)
