Protein moonlighting
Protein moonlighting is a phenomenon by which a protein can perform more than one function.[2] It is an excellent example of gene sharing.[3]
Ancestral moonlighting proteins originally possessed a single function but, through evolution, acquired additional functions. Many proteins that moonlight are enzymes; others are receptors, ion channels or chaperones. 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.[4]
Protein moonlighting occurs widely in nature.
Various techniques have been used to reveal moonlighting functions in proteins. The detection of a protein in unexpected locations within cells, cell types, or tissues may suggest that a protein has a moonlighting function. Furthermore, the sequence or structure homology of a protein may be used to infer both primary functions as well as secondary moonlighting functions of a protein.
The most well-studied examples of gene sharing are crystallins. These proteins, when expressed at low levels in many tissues function as enzymes, but when expressed at high levels in eye tissue, become densely packed and thus form lenses. While the recognition of gene sharing is relatively recent—the term was coined in 1988, after crystallins in chickens and ducks were found to be identical to separately identified enzymes—recent studies have found many examples throughout the living world. Joram Piatigorsky has suggested that many or all proteins exhibit gene sharing to some extent, and that gene sharing is a key aspect of molecular evolution.[9]: 1–7 The genes encoding crystallins must maintain sequences for catalytic function and transparency maintenance function.[8]
Inappropriate moonlighting is a contributing factor in some genetic diseases, and moonlighting provides a possible mechanism by which bacteria may become resistant to antibiotics.[10]
Discovery
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. Piatigorsky determined that lens crystallin conservation and variance are due to other moonlighting functions outside of the lens.[11] Originally Piatigorsky called these proteins "gene sharing" proteins, but the colloquial description moonlighting was subsequently applied to proteins by Constance Jeffery in 1999[12] to draw a similarity between multitasking proteins and people who work two jobs.[13] The phrase "gene sharing" is ambiguous since it is also used to describe horizontal gene transfer, hence the phrase "protein moonlighting" has become the preferred description for proteins with more than one function.[13]
Evolution
It is believed that moonlighting proteins came about by means of evolution through which uni-functional proteins gained the ability to perform multiple functions. With alterations, much of the protein's unused space can provide new functions.[10] Many moonlighting proteins are the result of the gene fusion of two single function genes.[14] Alternatively a single gene can acquire a second function since the active site of the encoded protein typically is small compared to the overall size of the protein leaving considerable room to accommodate a second functional site. In yet a third alternative, the same active site can acquire a second function through mutations of the active site.
The development of moonlighting proteins may be evolutionarily favorable to the organism since a single protein can do the job of multiple proteins conserving amino acids and energy required to synthesize these proteins.
Functions
Many proteins
A number of the currently known moonlighting proteins are evolutionarily derived from highly conserved enzymes, also called ancient enzymes. These enzymes are frequently speculated to have evolved moonlighting functions. 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. Furthermore, it has been suggested that as many as 7 out of 10 proteins in glycolysis and 7 out of 8 enzymes of the tricarboxylic acid cycle exhibit moonlighting behavior.[4]
An example of a moonlighting enzyme is
The E. coli anti-oxidant thioredoxin protein is another example of a moonlighting protein. Upon infection with the bacteriophage T7, E. coli thioredoxin forms a complex with T7 DNA polymerase, which results in enhanced T7 DNA replication, a crucial step for successful T7 infection. Thioredoxin binds to a loop in T7 DNA polymerase to bind more strongly to the DNA. The anti-oxidant function of thioredoxin is fully autonomous and fully independent of T7 DNA replication, in which the protein most likely fulfills the functional role.[13]
ADT2 and ADT5 are other examples of moonlighting proteins found in plants. Both of these proteins have roles in phenylalanine biosynthesis like all other ADTs. However ADT2, together with FtsZ is necessary in chloroplast division and ADT5 is transported by stromules into the nucleus.[15]
Examples
Kingdom | Protein | Organism | Function | |
---|---|---|---|---|
primary | moonlighting | |||
Animal | ||||
Aconitase | H. sapiens |
TCA cycle enzyme | Iron homeostasis | |
ATF2 |
H. sapiens |
Transcription factor | DNA damage response | |
Clathrin | H. sapiens |
Membrane traffic | Mitotic spindle stability | |
Crystallins |
Various | Lens structural protein | Various enzyme | |
Cytochrome c | Various | Energy metabolism | Apoptosis | |
DLD | H. sapiens | Energy metabolism | Protease | |
ERK2 |
H. sapiens |
MAP kinase | Transcriptional repressor | |
ESCRT-II complex | D. melanogaster | Endosomal protein sorting | Bicoid mRNA localization | |
STAT3 | M. musculus |
Transcription factor | Electron transport chain | |
Histone H3 | X. laevis | DNA packaging | Copper reductase[16] | |
Plant | ||||
Hexokinase | A. thaliana | Glucose metabolism | Glucose signaling/cell death control[17] | |
Presenilin | P. patens | γ-secretase | Cystoskeletal function | |
Fungus | ||||
Aconitase | S. cerevisiae | TCA cycle enzyme | mtDNA stability | |
Aldolase |
S. cerevisiae | Glycolytic enzyme | V-ATPase assembly | |
Arg5,6 | S. cerevisiae | Arginine biosynthesis | Transcriptional control | |
Enolase | S. cerevisiae | Glycolytic enzyme |
| |
Galactokinase | K. lactis | Galactose catabolism enzyme | Induction galactose genes | |
Hal3 | S. cerevisiae | Halotolerance determinant | Coenzyme A biosynthesis | |
HSP60 |
S. cerevisiae | Mitochondrial chaperone | Stabilization active DNA ori's | |
Phosphofructokinase | P. pastoris |
Glycolytic enzyme | Autophagy peroxisomes | |
Pyruvate carboxylase | H. polymorpha |
Anaplerotic enzyme | Assembly of alcohol oxidase | |
Vhs3 | S. cerevisiae | Halotolerance determinant | Coenzyme A biosynthesis | |
Prokaryotes | ||||
Aconitase | M. tuberculosis | TCA cycle enzyme | Iron-responsive protein | |
CYP170A1 |
S. coelicolor |
Albaflavenone synthase | Terpene synthase | |
Enolase | S. pneumoniae | Glycolytic enzyme | Plasminogen binding | |
GroEL | E. aerogenes |
Chaperone | Insect toxin | |
Glutamate racemase (MurI) | E. coli | cell wall biosynthesis | gyrase inhibition | |
Thioredoxin | E. coli | Anti-oxidant | T7 DNA polymerase subunit | |
Protist | ||||
Aldolase |
P. vivax | Glycolytic enzyme | Host-cell invasion |
Mechanisms
In many cases, the functionality of a protein not only depends on its structure, but also its location. For example, a single protein may have one function when found in the cytoplasm of a cell, a different function when interacting with a membrane, and yet a third function if excreted from the cell. This property of moonlighting proteins is known as "differential localization".
Other methods through which proteins may moonlight are by changing their
The crystal structures of several moonlighting proteins, such as I-AniI
Identification methods
Moonlighting proteins have usually been identified by chance because there is no clear procedure to identify secondary moonlighting functions. Despite such difficulties, the number of moonlighting proteins that have been discovered is rapidly increasing. Furthermore, moonlighting proteins appear to be abundant in all kingdoms of life.[13]
Various methods have been employed to determine a protein's function including secondary moonlighting functions. For example, the tissue, cellular, or subcellular distribution of a protein may provide hints as to the function. Real-time PCR is used to quantify mRNA and hence infer the presence or absence of a particular protein which is encoded by the mRNA within different cell types. Alternatively immunohistochemistry or mass spectrometry can be used to directly detect the presence of proteins and determine in which subcellular locations, cell types, and tissues a particular protein is expressed.
Mass spectrometry may be used to detect proteins based on their
The
Higher order multifunctionality
In the case of the glycolytic enzyme
Crystallins
In the case of
Gene regulation
Crystallin recruitment may occur by changes in
Alpha crystallins
The α-crystallins, which contributed to the discovery of crystallins as borrowed proteins,
Beta/gamma-crystallins
β/γ-crystallins are different from α-crystallins in that they are a large multigene family. Other proteins like bacterial spore coat, a slime mold cyst protein, and epidermis differentiation-specific protein, contain the same Greek key motifs and are placed under β/γ crystallin superfamily. This relationship supports the idea that β/γ- crystallins have been recruited by a gene-sharing mechanism. However, except for few reports, non-refractive function of the β/γ-crystallin is yet to be found.[30]
Corneal crystallins
Similar to
Non refractive roles of crystallins in lens and cornea
While it is evident that gene sharing resulted in many of lens crystallins being multifunctional proteins, it is still uncertain to what extent the crystallins use their non-refractive properties in the lens, or on what basis they were selected. The α-crystallins provide a convincing case for a lens crystallin using its non-refractive ability within the lens to prevent protein aggregation under a variety of environmental stresses
Co-evolution of lens and cornea through gene sharing
Based on the similarities between lens and cornea, such as abundant water-soluble enzymes, and being derived from ectoderm, the lens and cornea are thought to be co-evolved as a "refraction unit." Gene sharing would maximize light transmission and refraction to the retina by this refraction unit. 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. Also, the
Relationship to similar concepts
Gene sharing is related to, but distinct from, several concepts in genetics, evolution, and molecular biology. Gene sharing entails multiple effects from the same gene, but unlike
Clinical significance
The multiple roles of moonlighting proteins complicates the determination of
The complex phenotypes of several disorders are suspected to be caused by the involvement of moonlighting proteins. The protein GAPDH has at least 11 documented functions, one of which includes apoptosis. Excessive apoptosis is involved in many neurodegenerative diseases, such as Huntington's, Alzheimer's, and Parkinson's as well as in brain ischemia. In one case, GAPDH was found in the degenerated neurons of individuals who had Alzheimer's disease.[4]
Although there is insufficient evidence for definite conclusions, there are well documented examples of moonlighting proteins that play a role in disease. One such disease is
See also
- Enzyme promiscuity
- Pseudoenzymes
External links
- Media related to Moonlighting proteins at Wikimedia Commons
- moonlightingproteins.org database
References
- PMID 19858213.
- PMID 12902157.
- S2CID 42033757.
- ^ PMID 15877277.
- S2CID 254082728.
- PMID 31032837.
- PMID 36117533.
- ^ S2CID 37453649.
- ^ ISBN 978-0-674-02341-3.
- ^ PMID 18757813.
- PMID 3368457.
- ^ PMID 10087914.
- ^ PMID 20144902.
- PMID 18322039.
- PMID 28338876.
- S2CID 220304739.
- PMID 1560927.
- PMID 8151704.
- ^ PMID 15605385.
- PMID 22851445.
- S2CID 9917899.
- PMID 25399609.
- PMID 14633971.
- PMID 12514740.
- ^ PMID 15582389.
- PMID 20399282.
- PMID 25399609.
- ^ Harding JJ, Crabbe MJC (1984). "The lens: development, proteins, metabolism and cataract". In Davson H (ed.). The Eye. Vol. IB (3 ed.). New York: Academic Press. pp. 207–492.
- ^ PMID 3052280.
- ^ S2CID 8335681.
- S2CID 30002410.
- PMID 6285380.
- PMID 8450753.
- PMID 8441415.
- PMID 8638673.
- PMID 2912453.
- PMID 1932094.
- PMID 2725488.
- PMID 4630510.
- PMID 8260946.
- PMID 7909809.
- PMID 8727959.
- S2CID 21387795.
- PMID 2053490.
- PMID 17997336.
- PMID 25163484.