Protein moonlighting
The most common primary function of moonlighting proteins is enzymatic catalysis, but these enzymes have acquired secondary non-enzymatic roles.
Some examples of functions of moonlighting proteins secondary to catalysis include signal transduction, transcriptional regulation, apoptosis, motility, and structural.
[10] The first observation of a moonlighting protein was made in the late 1980s by Joram Piatigorsky and Graeme Wistow during their research on crystallin enzymes.
[12][13] While using one protein to perform multiple roles seems advantageous because it keeps the genome small, we can conclude that this is probably not the reason for moonlighting because of the large amount of noncoding DNA.
Since highly conserved proteins are present in many different organisms, this increases the chance that they would develop secondary moonlighting functions.
[13] A high fraction of enzymes involved in glycolysis, an ancient universal metabolic pathway, exhibit moonlighting behavior.
Surprisingly, in yeast species such as H. polymorpha and P. pastoris, pyruvate carboylase is also essential for the proper targeting and assembly of the peroxisomal protein alcohol oxidase (AO).
In wild type cells, this enzyme is present as enzymatically active AO octamers in the peroxisomal matrix.
Conversely, mutations are known that block the function of this enzyme in the import and assembly of AO, but have no effect on the enzymatic activity of the protein.
[20] In the case of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) alterations in the PTMs have been shown to be associated with higher order multi functionality.
An example of this is ceruloplasmin, a protein that functions as an oxidase in copper metabolism and moonlights as a copper-independent glutathione peroxidase.
[13] The crystal structures of several moonlighting proteins, such as I-AniI homing endonuclease / maturase[23] and the PutA proline dehydrogenase / transcription factor,[24] have been determined.
As seen in ƞ-crystallin, the second function of a protein can alter the structure, decreasing the flexibility, which in turn can impair enzymatic activity somewhat.
Because of alternative splicing and posttranslational modification, identification of proteins based on the mass of the parent ion alone is very difficult.
Hence tandem mass spectrometry is one of the tools used in proteomics to identify the presence of proteins in different cell types or subcellular locations.
[19] Furthermore, mass spectrometry may be used to determine if a protein has high expression levels that do not correlate to the enzyme's measured metabolic activity.
Finally, application of systems biology approaches[26] such as interactomics give clues to a proteins function based on what it interacts with.
Moreover, in case of its iron import activities it can traffic into cells holo-transferrin as well as the related molecule lactoferrin by multiple pathways.
[8] The abundant lens crystallins have been generally viewed as static proteins serving a strictly structural role in transparency and cataract.
[28] However, recent studies have shown that the lens crystallins are much more diverse than previously recognized and that many are related or identical to metabolic enzymes and stress proteins found in numerous tissues.
[29] Unlike other proteins performing highly specialized tasks, such as globin or rhodopsin, the crystallins are very diverse and show numerous species differences.
[35] αB-crystallin is also overexpressed in many pathologies, including neurodegenerative diseases, fibroblasts of patients with Werner syndrome showing premature senescence, and growth abnormalities.
In addition to being overexpressed under abnormal conditions, αB-crystallin is constitutively expressed in heart, skeletal muscle, kidney, lung and many other tissues.
[30] Similar to lens, cornea is a transparent, avascular tissue derived from the ectoderm that is responsible for focusing light onto the retina.
Early immunology studies have shown that BCP 54 comprises 20–40% of the total soluble protein in bovine cornea.
[39] Subsequent studies have indicated that BCP 54 is ALDH3, a tumor and xenobiotic-inducible cytosolic enzyme, found in human, rat, and other mammals.
[42] The α-crystallins may also play a functional role in the stability and remodeling of the cytoskeleton during fiber cell differentiation in the lens.
Studies have shown that many water-soluble enzymes/proteins expressed by cornea are identical to taxon-specific lens crystallins, such as ALDH1A1/ η-crystallin, α-enolase/τ-crystallin, and lactic dehydrogenase/ -crystallin.
[9]: 8–14 The multiple roles of moonlighting proteins complicates the determination of phenotype from genotype,[4] hampering the study of inherited metabolic disorders.
[4] Although there is insufficient evidence for definite conclusions, there are well documented examples of moonlighting proteins that play a role in disease.
