ADAR

The double-stranded RNA-specific adenosine deaminase enzyme family are encoded by the ADAR family genes. ADAR stands for adenosine deaminase acting on RNA. This article focuses on the ADAR proteins; This article details the evolutionary history, structure, function, mechanisms and importance of all proteins within this family.

ADAR enzymes bind to double-stranded RNA (dsRNA) and convert adenosine to inosine (hypoxanthine) by deamination. ADAR proteins act post-transcriptionally, changing the nucleotide content of RNA. The conversion from adenosine to inosine (A to I) in the RNA disrupts the normal A:U pairing, destabilizing the RNA. Inosine is structurally similar to guanine (G) which leads to inosine to cytosine (I:C) binding. Inosine typically mimics guanosine during translation but can also bind to uracil, cytosine, and adenosine, though it is not favored.

Codon changes may arise from RNA editing leading to changes in the coding sequences for proteins and their functions. Most editing sites are found in noncoding regions of RNA such as untranslated regions (UTRs), Alu elements, and long interspersed nuclear elements (LINEs). Codon changes can give rise to alternate transcriptional splice variants. ADAR impacts the transcriptome in editing-independent ways, likely by interfering with other RNA-binding proteins.

Mutations in this gene are associated with several diseases including HIV, measles, and melanoma. Recent research supports a linkage between RNA-editing and nervous system disorders such as amyotrophic lateral sclerosis (ALS). Atypical RNA editing linked to ADAR may also correlate to mental disorders such as schizophrenia, epilepsy, and suicidal depression.

The ADAR enzyme and its associated gene were discovered accidentally in 1987 as a result of research by Brenda Bass and Harold Weintraub. These researchers were using antisense RNA inhibition to determine which genes play a key role in the development of Xenopus laevis embryos. Previous research on Xenopus oocytes was successful. However, when Bass and Weintraub applied identical protocols to Xenopus embryos, they were unable to determine the embryo's developmental genes. To understand why the method was unsuccessful, they began comparing duplex RNA in both oocytes and embryos. This led them to discover a developmentally regulated activity that denatures RNA:RNA hybrids in embryos.

In 1988, Richard Wagner et al. further studied the activity occurring on Xenopus embryos. They determined a protein was responsible for unwinding of RNA due to the absence of activity after proteinase treatment. This protein is specific for dsRNA and does not require ATP. It became evident this protein's activity on dsRNA modifies it beyond a point of rehybridization but does not fully denature it. Finally, the researchers determined this unwinding is due to the deamination of adenosine residues to inosine. This modification results in mismatched base-pairing between inosine and uridine, leading to the destabilization and unwinding of dsRNA.

ADARs are one of the most common forms of RNA editing, and have both selective and non-selective activity. ADAR is able to modify and regulate the output of gene product, as inosine is interpreted by the cell to be guanosine. ADAR can change the functionality of small RNA molecules. Recently, ADARs have also been discovered as a regulator on splicing and circRNA biogenesis with their editing capability or RNA binding function. It is believed that ADAR evolved from ADAT (Adenosine Deaminase Acting on tRNA), a critical protein present in all eukaryotes, early in the metazoan period through the addition of a dsRNA binding domain. This likely occurred in the lineage which leads to the crown Metazoa. When a duplicate ADAT gene was coupled to another gene which encoded at least one double stranded RNA binding. The ADAR family of genes has been largely conserved over the history of its existence. This, along with its presence in the majority of modern phyla, indicates that RNA editing is essential in regulating genes for metazoan organisms. ADAR has not been discovered in a variety of non-metazoan eukaryotes, such as plants, fungi and choanoflagellates.

ADARs are suggested to have two functions: to increase diversity of the proteome by inducing creation of harmless non-genomically encoded proteins, and protecting crucial translational sites. The conventional belief is their primary role is to increase the diversity of transcripts and expand the protein variation, promoting evolution of proteins.