In
molecular biology, a two-component regulatory system serves as a basic stimulus-response coupling mechanism to allow
organisms to
sense and respond to changes in many different environmental conditions.[1] Two-component systems typically consist of a
membrane-bound
histidine kinase that senses a specific environmental
stimulus, and a corresponding
response regulator that mediates the cellular response, mostly through differential
expression of target
genes.[2] Although two-component signaling systems are found in all
domains of life, they are most common by far in
bacteria, particularly in
Gram-negative and
cyanobacteria; both histidine kinases and response regulators are among the largest gene families in bacteria.[3] They are much less common in
archaea and
eukaryotes; although they do appear in
yeasts,
filamentous fungi, and
slime molds, and are common in
plants,[1] two-component systems have been described as "conspicuously absent" from
animals.[3]
Mechanism
Two-component systems accomplish
signal transduction through the
phosphorylation of a
response regulator (RR) by a
histidine kinase (HK). Histidine kinases are typically
homodimerictransmembrane proteins containing a histidine phosphotransfer
domain and an ATP binding domain, though there are reported examples of histidine kinases in the atypical
HWE and
HisKA2 families that are not homodimers.[4] Response regulators may consist only of a receiver domain, but usually are multi-domain proteins with a receiver domain and at least one effector or output domain, often involved in
DNA binding.[3] Upon detecting a particular change in the extracellular environment, the HK performs an
autophosphorylation reaction, transferring a
phosphoryl group from
adenosine triphosphate (ATP) to a specific
histidine residue. The cognate
response regulator (RR) then catalyzes the transfer of the phosphoryl group to an
aspartate residue on the response regulator's receiver
domain.[5][6] This typically triggers a
conformational change that activates the RR's effector domain, which in turn produces the cellular response to the signal, usually by stimulating (or repressing)
expression of target
genes.[3]
Many HKs are bifunctional and possess
phosphatase activity against their cognate response regulators, so that their signaling output reflects a balance between their kinase and phosphatase activities. Many response regulators also auto-dephosphorylate,[7] and the relatively labile phosphoaspartate can also be hydrolyzed non-enzymatically.[1] The overall level of
phosphorylation of the response regulator ultimately controls its activity.[1][8]
Phosphorelays
Some histidine kinases are hybrids that contain an internal receiver domain. In these cases, a hybrid HK autophosphorylates and then transfers the phosphoryl group to its own internal receiver domain, rather than to a separate RR protein. The phosphoryl group is then shuttled to
histidine phosphotransferase (HPT) and subsequently to a terminal RR, which can evoke the desired response.[9][10] This system is called a phosphorelay. Almost 25% of bacterial HKs are of the hybrid type, as are the large majority of eukaryotic HKs.[3]
Function
Two-component
signal transduction systems enable
bacteria to sense, respond, and adapt to a wide range of environments, stressors, and
growth conditions.[11] These pathways have been adapted to respond to a wide variety of stimuli, including
nutrients,
cellularredox state, changes in
osmolarity,
quorum signals,
antibiotics,
temperature,
chemoattractants,
pH and more.[12][13] The average number of two-component systems in a bacterial
genome has been estimated as around 30,[14] or about 1–2% of a prokaryote's genome.[15] A few bacteria have none at all – typically endosymbionts and pathogens – and others contain over 200.[16][17] All such systems must be closely
regulated to prevent cross-talk, which is rare in vivo.[18]
Signal transducing
histidine kinases are the key elements in two-component signal transduction systems.[22][23] Examples of histidine kinases are EnvZ, which plays a central role in
osmoregulation,[24] and CheA, which plays a central role in the
chemotaxis system.[25] Histidine kinases usually have an
N-terminalligand-binding domain and a
C-terminal kinase domain, but other
domains may also be present. The kinase domain is responsible for the autophosphorylation of the histidine with ATP, the phosphotransfer from the kinase to an
aspartate of the response regulator, and (with bifunctional enzymes) the phosphotransfer from
aspartyl phosphate to
water.[26] The kinase core has a unique fold, distinct from that of the Ser/Thr/Tyr kinase
superfamily.
HKs can be roughly divided into two classes: orthodox and hybrid kinases.[27][28] Most orthodox HKs, typified by the E. coli EnvZ protein, function as
periplasmic membrane receptors and have a
signal peptide and
transmembrane segment(s) that separate the protein into a periplasmic N-terminal sensing domain and a highly conserved
cytoplasmic C-terminal kinase core. Members of this family, however, have an integral membrane sensor domain. Not all orthodox kinases are
membrane bound, e.g., the
nitrogen regulatory kinase NtrB (GlnL) is a soluble
cytoplasmic HK.[6] Hybrid kinases contain multiple phosphodonor and phosphoacceptor sites and use multi-step phospho-relay schemes instead of promoting a single phosphoryl transfer. In addition to the sensor domain and kinase core, they contain a CheY-like receiver domain and a His-containing phosphotransfer (HPt) domain.
Evolution
The number of two-component systems present in a bacterial genome is highly correlated with genome size as well as
ecological niche; bacteria that occupy niches with frequent environmental fluctuations possess more histidine kinases and response regulators.[3][29] New two-component systems may arise by
gene duplication or by
lateral gene transfer, and the relative rates of each process vary dramatically across bacterial species.[30] In most cases, response regulator genes are located in the same
operon as their cognate histidine kinase;[3] lateral gene transfers are more likely to preserve operon structure than gene duplications.[30]
In eukaryotes
Two-component systems are rare in
eukaryotes. They appear in
yeasts,
filamentous fungi, and
slime molds, and are relatively common in
plants, but have been described as "conspicuously absent" from
animals.[3] Two-component systems in eukaryotes likely originate from
lateral gene transfer, often from
endosymbiotic organelles, and are typically of the hybrid kinase phosphorelay type.[3] For example, in the yeast Candida albicans, genes found in the nuclear genome likely originated from
endosymbiosis and remain targeted to the
mitochondria.[31] Two-component systems are well-integrated into developmental signaling pathways in plants, but the genes probably originated from
lateral gene transfer from
chloroplasts.[3] An example is the
chloroplast sensor kinase (CSK) gene in Arabidopsis thaliana, derived from chloroplasts but now integrated into the nuclear genome. CSK function provides a
redox-based regulatory system that couples
photosynthesis to chloroplast
gene expression; this observation has been described as a key prediction of the
CoRR hypothesis, which aims to explain the retention of genes encoded by endosymbiotic organelles.[32][33]
It is unclear why canonical two-component systems are rare in eukaryotes, with many similar functions having been taken over by signaling systems based on
serine,
threonine, or
tyrosine kinases; it has been speculated that the chemical instability of phosphoaspartate is responsible, and that increased stability is needed to transduce signals in the more complex eukaryotic cell.[3] Notably, cross-talk between signaling mechanisms is very common in eukaryotic signaling systems but rare in bacterial two-component systems.[34]
Bioinformatics
Because of their
sequence similarity and
operon structure, many two-component systems – particularly histidine kinases – are relatively easy to identify through
bioinformatics analysis. (By contrast, eukaryotic kinases are typically easily identified, but they are not easily paired with their
substrates.)[3] A
database of prokaryotic two-component systems called P2CS has been compiled to document and classify known examples, and in some cases to make predictions about the cognates of "orphan" histidine kinase or response regulator proteins that are genetically unlinked to a partner.[35][36]
^Varughese KI (Apr 2002). "Molecular recognition of bacterial phosphorelay proteins". Current Opinion in Microbiology. 5 (2): 142–8.
doi:
10.1016/S1369-5274(02)00305-3.
PMID11934609.
^Perego M, Hoch JA (Mar 1996). "Protein aspartate phosphatases control the output of two-component signal transduction systems". Trends in Genetics. 12 (3): 97–101.
doi:
10.1016/0168-9525(96)81420-X.
PMID8868347.
^West AH, Stock AM (Jun 2001). "Histidine kinases and response regulator proteins in two-component signaling systems". Trends in Biochemical Sciences. 26 (6): 369–76.
doi:
10.1016/S0968-0004(01)01852-7.
PMID11406410.
^Tomomori C, Tanaka T, Dutta R, Park H, Saha SK, Zhu Y, Ishima R, Liu D, Tong KI, Kurokawa H, Qian H, Inouye M, Ikura M (Aug 1999). "Solution structure of the homodimeric core domain of Escherichia coli histidine kinase EnvZ". Nature Structural Biology. 6 (8): 729–34.
doi:
10.1038/11495.
PMID10426948.
S2CID23334643.
^Vierstra RD, Davis SJ (Dec 2000). "Bacteriophytochromes: new tools for understanding phytochrome signal transduction". Seminars in Cell & Developmental Biology. 11 (6): 511–21.
doi:
10.1006/scdb.2000.0206.
PMID11145881.
^Alex LA, Simon MI (Apr 1994). "Protein histidine kinases and signal transduction in prokaryotes and eukaryotes". Trends in Genetics. 10 (4): 133–8.
doi:
10.1016/0168-9525(94)90215-1.
PMID8029829.
In
molecular biology, a two-component regulatory system serves as a basic stimulus-response coupling mechanism to allow
organisms to
sense and respond to changes in many different environmental conditions.[1] Two-component systems typically consist of a
membrane-bound
histidine kinase that senses a specific environmental
stimulus, and a corresponding
response regulator that mediates the cellular response, mostly through differential
expression of target
genes.[2] Although two-component signaling systems are found in all
domains of life, they are most common by far in
bacteria, particularly in
Gram-negative and
cyanobacteria; both histidine kinases and response regulators are among the largest gene families in bacteria.[3] They are much less common in
archaea and
eukaryotes; although they do appear in
yeasts,
filamentous fungi, and
slime molds, and are common in
plants,[1] two-component systems have been described as "conspicuously absent" from
animals.[3]
Mechanism
Two-component systems accomplish
signal transduction through the
phosphorylation of a
response regulator (RR) by a
histidine kinase (HK). Histidine kinases are typically
homodimerictransmembrane proteins containing a histidine phosphotransfer
domain and an ATP binding domain, though there are reported examples of histidine kinases in the atypical
HWE and
HisKA2 families that are not homodimers.[4] Response regulators may consist only of a receiver domain, but usually are multi-domain proteins with a receiver domain and at least one effector or output domain, often involved in
DNA binding.[3] Upon detecting a particular change in the extracellular environment, the HK performs an
autophosphorylation reaction, transferring a
phosphoryl group from
adenosine triphosphate (ATP) to a specific
histidine residue. The cognate
response regulator (RR) then catalyzes the transfer of the phosphoryl group to an
aspartate residue on the response regulator's receiver
domain.[5][6] This typically triggers a
conformational change that activates the RR's effector domain, which in turn produces the cellular response to the signal, usually by stimulating (or repressing)
expression of target
genes.[3]
Many HKs are bifunctional and possess
phosphatase activity against their cognate response regulators, so that their signaling output reflects a balance between their kinase and phosphatase activities. Many response regulators also auto-dephosphorylate,[7] and the relatively labile phosphoaspartate can also be hydrolyzed non-enzymatically.[1] The overall level of
phosphorylation of the response regulator ultimately controls its activity.[1][8]
Phosphorelays
Some histidine kinases are hybrids that contain an internal receiver domain. In these cases, a hybrid HK autophosphorylates and then transfers the phosphoryl group to its own internal receiver domain, rather than to a separate RR protein. The phosphoryl group is then shuttled to
histidine phosphotransferase (HPT) and subsequently to a terminal RR, which can evoke the desired response.[9][10] This system is called a phosphorelay. Almost 25% of bacterial HKs are of the hybrid type, as are the large majority of eukaryotic HKs.[3]
Function
Two-component
signal transduction systems enable
bacteria to sense, respond, and adapt to a wide range of environments, stressors, and
growth conditions.[11] These pathways have been adapted to respond to a wide variety of stimuli, including
nutrients,
cellularredox state, changes in
osmolarity,
quorum signals,
antibiotics,
temperature,
chemoattractants,
pH and more.[12][13] The average number of two-component systems in a bacterial
genome has been estimated as around 30,[14] or about 1–2% of a prokaryote's genome.[15] A few bacteria have none at all – typically endosymbionts and pathogens – and others contain over 200.[16][17] All such systems must be closely
regulated to prevent cross-talk, which is rare in vivo.[18]
Signal transducing
histidine kinases are the key elements in two-component signal transduction systems.[22][23] Examples of histidine kinases are EnvZ, which plays a central role in
osmoregulation,[24] and CheA, which plays a central role in the
chemotaxis system.[25] Histidine kinases usually have an
N-terminalligand-binding domain and a
C-terminal kinase domain, but other
domains may also be present. The kinase domain is responsible for the autophosphorylation of the histidine with ATP, the phosphotransfer from the kinase to an
aspartate of the response regulator, and (with bifunctional enzymes) the phosphotransfer from
aspartyl phosphate to
water.[26] The kinase core has a unique fold, distinct from that of the Ser/Thr/Tyr kinase
superfamily.
HKs can be roughly divided into two classes: orthodox and hybrid kinases.[27][28] Most orthodox HKs, typified by the E. coli EnvZ protein, function as
periplasmic membrane receptors and have a
signal peptide and
transmembrane segment(s) that separate the protein into a periplasmic N-terminal sensing domain and a highly conserved
cytoplasmic C-terminal kinase core. Members of this family, however, have an integral membrane sensor domain. Not all orthodox kinases are
membrane bound, e.g., the
nitrogen regulatory kinase NtrB (GlnL) is a soluble
cytoplasmic HK.[6] Hybrid kinases contain multiple phosphodonor and phosphoacceptor sites and use multi-step phospho-relay schemes instead of promoting a single phosphoryl transfer. In addition to the sensor domain and kinase core, they contain a CheY-like receiver domain and a His-containing phosphotransfer (HPt) domain.
Evolution
The number of two-component systems present in a bacterial genome is highly correlated with genome size as well as
ecological niche; bacteria that occupy niches with frequent environmental fluctuations possess more histidine kinases and response regulators.[3][29] New two-component systems may arise by
gene duplication or by
lateral gene transfer, and the relative rates of each process vary dramatically across bacterial species.[30] In most cases, response regulator genes are located in the same
operon as their cognate histidine kinase;[3] lateral gene transfers are more likely to preserve operon structure than gene duplications.[30]
In eukaryotes
Two-component systems are rare in
eukaryotes. They appear in
yeasts,
filamentous fungi, and
slime molds, and are relatively common in
plants, but have been described as "conspicuously absent" from
animals.[3] Two-component systems in eukaryotes likely originate from
lateral gene transfer, often from
endosymbiotic organelles, and are typically of the hybrid kinase phosphorelay type.[3] For example, in the yeast Candida albicans, genes found in the nuclear genome likely originated from
endosymbiosis and remain targeted to the
mitochondria.[31] Two-component systems are well-integrated into developmental signaling pathways in plants, but the genes probably originated from
lateral gene transfer from
chloroplasts.[3] An example is the
chloroplast sensor kinase (CSK) gene in Arabidopsis thaliana, derived from chloroplasts but now integrated into the nuclear genome. CSK function provides a
redox-based regulatory system that couples
photosynthesis to chloroplast
gene expression; this observation has been described as a key prediction of the
CoRR hypothesis, which aims to explain the retention of genes encoded by endosymbiotic organelles.[32][33]
It is unclear why canonical two-component systems are rare in eukaryotes, with many similar functions having been taken over by signaling systems based on
serine,
threonine, or
tyrosine kinases; it has been speculated that the chemical instability of phosphoaspartate is responsible, and that increased stability is needed to transduce signals in the more complex eukaryotic cell.[3] Notably, cross-talk between signaling mechanisms is very common in eukaryotic signaling systems but rare in bacterial two-component systems.[34]
Bioinformatics
Because of their
sequence similarity and
operon structure, many two-component systems – particularly histidine kinases – are relatively easy to identify through
bioinformatics analysis. (By contrast, eukaryotic kinases are typically easily identified, but they are not easily paired with their
substrates.)[3] A
database of prokaryotic two-component systems called P2CS has been compiled to document and classify known examples, and in some cases to make predictions about the cognates of "orphan" histidine kinase or response regulator proteins that are genetically unlinked to a partner.[35][36]
^Varughese KI (Apr 2002). "Molecular recognition of bacterial phosphorelay proteins". Current Opinion in Microbiology. 5 (2): 142–8.
doi:
10.1016/S1369-5274(02)00305-3.
PMID11934609.
^Perego M, Hoch JA (Mar 1996). "Protein aspartate phosphatases control the output of two-component signal transduction systems". Trends in Genetics. 12 (3): 97–101.
doi:
10.1016/0168-9525(96)81420-X.
PMID8868347.
^West AH, Stock AM (Jun 2001). "Histidine kinases and response regulator proteins in two-component signaling systems". Trends in Biochemical Sciences. 26 (6): 369–76.
doi:
10.1016/S0968-0004(01)01852-7.
PMID11406410.
^Tomomori C, Tanaka T, Dutta R, Park H, Saha SK, Zhu Y, Ishima R, Liu D, Tong KI, Kurokawa H, Qian H, Inouye M, Ikura M (Aug 1999). "Solution structure of the homodimeric core domain of Escherichia coli histidine kinase EnvZ". Nature Structural Biology. 6 (8): 729–34.
doi:
10.1038/11495.
PMID10426948.
S2CID23334643.
^Vierstra RD, Davis SJ (Dec 2000). "Bacteriophytochromes: new tools for understanding phytochrome signal transduction". Seminars in Cell & Developmental Biology. 11 (6): 511–21.
doi:
10.1006/scdb.2000.0206.
PMID11145881.
^Alex LA, Simon MI (Apr 1994). "Protein histidine kinases and signal transduction in prokaryotes and eukaryotes". Trends in Genetics. 10 (4): 133–8.
doi:
10.1016/0168-9525(94)90215-1.
PMID8029829.