AARS - Alanine-TRNA Ligase |Elisa - Clia - Antibody - Protein

Background:

Alanine-tRNA ligase, encoded by the genes AARS1 (cytoplasmic) and AARS2 (mitochondrial), is a member of the aminoacyl-tRNA synthetase (aaRS) family. These enzymes are crucial for protein synthesis, catalyzing the attachment of alanine to its corresponding tRNA (tRNA^Ala) in a process known as tRNA aminoacylation. This reaction is an essential step in the translation process, ensuring accurate incorporation of alanine into nascent polypeptides according to the genetic code.

AARS1 and AARS2 differ in their subcellular localization and roles:

• AARS1 functions in the cytoplasm and is involved in general protein synthesis.
• AARS2 operates in mitochondria, where it is critical for mitochondrial protein synthesis, essential for maintaining respiratory chain function.

The genes encoding these enzymes are conserved across species, reflecting their indispensable role in cellular physiology. Mutations in these genes are associated with various diseases, including neurodegenerative disorders, mitochondrial dysfunction, and cancer.

Protein Structure

Primary Structure:

• AARS1 and AARS2 are large proteins, consisting of approximately 900–1000 amino acids.
• Both proteins exhibit conserved sequences in their catalytic and tRNA-binding domains, critical for their enzymatic functions.
• AARS2 includes an N-terminal mitochondrial targeting sequence (MTS), enabling its localization and function within mitochondria.

Domain Organization:

• Catalytic Domain:
• Responsible for adenylation and aminoacylation reactions.
• Binds ATP and alanine, forming an aminoacyl-adenylate intermediate.
• Transfers alanine to the 3’ hydroxyl group of the tRNA’s terminal adenosine.
• tRNA-Binding Domain:
• Specifically recognizes the anticodon loop and acceptor stem of tRNA^Ala, ensuring accurate aminoacylation.
• Editing Domain (Post-Transfer Editing):
• Hydrolyzes incorrectly charged tRNA (e.g., Ser-tRNA^Ala), safeguarding translational fidelity.
• Ensures prevention of misincorporation of amino acids into proteins.
• N-terminal Mitochondrial Targeting Signal (AARS2 only):
• Facilitates transport into mitochondria, where it undergoes proteolytic cleavage to activate the mature enzyme.

Tertiary and Quaternary Structure:

• AARS1 and AARS2 form monomeric structures with extensive inter-domain interactions.
• Their tertiary structures are stabilized by conserved folds, including Rossmann folds for ATP binding and alpha-beta sandwich motifs in the tRNA-binding domain.
• Structural studies have revealed that specific conformational changes facilitate substrate recognition and catalytic turnover.

Structural Variability:

• Although structurally homologous to other class II aminoacyl-tRNA synthetases, alanine-tRNA ligases exhibit unique structural features that enhance their substrate specificity for alanine and tRNA^Ala.

Classification and Subtypes

Alanine-tRNA ligases belong to the class II aminoacyl-tRNA synthetases, characterized by their conserved domain structure and catalytic mechanism. Within this class:

• AARS1 (cytoplasmic) and AARS2 (mitochondrial) are the two main subtypes in humans.
• Prokaryotic homologs share structural similarities but lack the mitochondrial targeting sequence found in AARS2.

Function and Biological Significance

Catalysis of Aminoacylation:

• AARS1 and AARS2 catalyze the attachment of alanine to tRNA^Ala in a two-step reaction:

1. Activation of alanine with ATP, forming alanine-adenylate.
2. Transfer of alanine to tRNA^Ala, forming alanine-charged tRNA.

• This charged tRNA is delivered to the ribosome during translation.

Quality Control and Editing:

• Both AARS1 and AARS2 possess proofreading mechanisms to ensure accurate amino acid charging.
• Misacylated tRNAs are hydrolyzed, preventing translational errors and misfolded proteins.

Mitochondrial Function (AARS2):

• AARS2 is essential for mitochondrial translation, playing a direct role in synthesizing components of the electron transport chain.
• Disruption of AARS2 affects mitochondrial function, leading to compromised oxidative phosphorylation and energy production.

Cellular Homeostasis:

• AARS1 and AARS2 maintain the fidelity of protein synthesis, preventing proteotoxic stress and ensuring proper cellular function.
• Both enzymes contribute to adaptive responses during stress, such as hypoxia or nutrient deprivation.

Clinical Issues

Genetic Mutations and Disorders:

• Mutations in AARS1 and AARS2 have been associated with neurodegenerative diseases and mitochondrial dysfunctions.
• AARS2 mutations cause a spectrum of mitochondrial disorders, including:
• Leukoencephalopathy with ovarian failure (LOF): A rare neurodegenerative condition characterized by white matter abnormalities and premature ovarian failure.
• Cardiomyopathy: Impaired mitochondrial function in cardiac muscle.
• Mutations in AARS1 are linked to axonal neuropathies, such as Charcot-Marie-Tooth disease.

Cancer:

• Dysregulation of AARS1 and AARS2 expression has been observed in certain cancers, potentially influencing tumor growth and metastasis through altered protein synthesis.

Mitochondrial Disorders:

• Deficiencies in AARS2 lead to reduced mitochondrial protein synthesis, impairing respiratory chain function and energy metabolism.

Inflammation and Immune Response:

• Aminoacyl-tRNA synthetases, including AARS1 and AARS2, have been implicated in non-canonical roles such as immune signaling, contributing to inflammatory responses under pathological conditions.

Summary

AARS1 and AARS2 encode cytoplasmic and mitochondrial alanine-tRNA ligases, respectively, essential for accurate protein synthesis. These enzymes catalyze the attachment of alanine to tRNA^Ala, ensuring the fidelity of translation. Their structures include catalytic, tRNA-binding, and editing domains, with AARS2 possessing an additional mitochondrial targeting sequence. Mutations in these genes cause diverse pathological conditions, including neurodegeneration, mitochondrial dysfunction, and cancer. Beyond translation, AARS1 and AARS2 have emerging roles in cellular homeostasis and immune regulation, underscoring their multifaceted significance in health and disease. Understanding the precise mechanisms and implications of these enzymes continues to reveal their crucial roles in cellular biology.