View source for Iron-starvation-induced protein A

{{Pfam_box
| Symbol = IsiA, CP43'
| Name = Iron-starvation-induced chlorophyll-a/b-binding protein
| Pfam= PF00421
| InterPro= IPR000932
}}
'''Iron-starvation-induced protein A''', also known as '''isiA''', is a photosynthesis-related chlorophyll-containing protein found in [[cyanobacteria]].  It belongs to the chlorophyll-a/b-binding family of proteins, and has been shown to have a [[photoprotective|photoprotection]] role in preventing oxidative damage via energy dissipation.  It was originally identified under Fe starvation, and thus received the name iron-starvation-induced protein A.  However, the protein has more recently been found to respond to a variety of stress conditions such as high irradiance.  It can aggregate with carotenoids and form rings around the PSI reaction center complexes to aid in photoprotective energy dissipation.<ref name="Bereravan2009" />
==Antenna function==
IsiA functions as an antenna for photosystem I (PSI) under iron-limiting conditions, when phycobilisomes disappear. In the (PSI)3(Isi3)18 complex most of the harvested energy is probably used by PSI; in other PSI-containing supercomplexes a large part of the energy will probably not be used for light harvesting, but rather is dissipated to protect the organism from light damage.
Under iron-starvation, it forms a complex with PSI trimers, where the trimer is surrounded by a ring composed of 18 isiA subunits.<ref name="Yeremenko2004"/> When the PSI subunits PsaF and PsaJ are missing the ring is composed of 17 isiA subunits, indicating that each isiA subunit has a different interaction with the trimer. This suggests that the size of the PSI complex determines the number of isiA units in the surrounding ring. In the absence of PsaL, it has a tendency to form incomplete rings with PSI monomers, suggesting PsaL helps form the rings. Also it can form other aggregates of varying sizes depending on the level of iron deprivation.
==Photoprotective function==
IsiA aggregates forming empty multimeric rings (without PSI) also accumulate and are very abundant in long-term iron-depleted cells.  When isolated, these aggregates are in a strongly quenched state,<ref name="Ihalainen2005" /> suggesting they are responsible for thermal dissipation of absorbed energy.<ref name="Wilson2007" />  IsiA is also synthesized in cells grown under high light irradiances, protecting from photodamage <ref name="Havaux2005" />
== See also ==
* [[Photosystem II light-harvesting protein]]
== References ==
{{Reflist|
refs=
<ref name="Bereravan2009">{{cite journal | vauthors = Berera R, van Stokkum IH, d'Haene S, Kennis JT, van Grondelle R, Dekker JP | title = A mechanism of energy dissipation in cyanobacteria | journal = Biophysical Journal | volume = 96 | issue = 6 | pages = 2261–7 | date = March 2009 | pmid = 19289052 | pmc = 2717300 | doi = 10.1016/j.bpj.2008.12.3905 | bibcode = 2009BpJ....96.2261B }}</ref>
<ref name="Yeremenko2004">{{cite journal | vauthors = Yeremenko N, Kouril R, Ihalainen JA, D'Haene S, van Oosterwijk N, Andrizhiyevskaya EG, Keegstra W, Dekker HL, Hagemann M, Boekema EJ, Matthijs HC, Dekker JP | title = Supramolecular organization and dual function of the IsiA chlorophyll-binding protein in cyanobacteria | journal = Biochemistry | volume = 43 | issue = 32 | pages = 10308–13 | date = August 2004 | pmid = 15301529 | doi = 10.1021/bi048772l | citeseerx = 10.1.1.516.3974 }}</ref>
<ref name="Wilson2007">{{cite journal | vauthors = Wilson A, Boulay C, Wilde A, Kerfeld CA, Kirilovsky D | title = Light-induced energy dissipation in iron-starved cyanobacteria: roles of OCP and IsiA proteins | journal = The Plant Cell | volume = 19 | issue = 2 | pages = 656–72 | date = February 2007 | pmid = 17307930 | pmc = 1867334 | doi = 10.1105/tpc.106.045351 }}</ref>
<ref name="Ihalainen2005">{{cite journal | vauthors = Ihalainen JA, D'Haene S, Yeremenko N, van Roon H, Arteni AA, Boekema EJ, van Grondelle R, Matthijs HC, Dekker JP | title = Aggregates of the chlorophyll-binding protein IsiA (CP43') dissipate energy in cyanobacteria | journal = Biochemistry | volume = 44 | issue = 32 | pages = 10846–53 | date = August 2005 | pmid = 16086587 | doi = 10.1021/bi0510680 | url = https://www.rug.nl/research/portal/files/3622555/2005BiochemIhalainen.pdf }}</ref>
<ref name="Havaux2005">{{cite journal | vauthors = Havaux M, Guedeney G, Hagemann M, Yeremenko N, Matthijs HC, Jeanjean R | title = The chlorophyll-binding protein IsiA is inducible by high light and protects the cyanobacterium Synechocystis PCC6803 from photooxidative stress | journal = FEBS Letters | volume = 579 | issue = 11 | pages = 2289–93 | date = April 2005 | pmid = 15848160 | doi = 10.1016/j.febslet.2005.03.021 | s2cid = 23867079 | doi-access = free }}</ref>
}}
== External links ==
* {{UniProt Full|Q55274|Iron stress-induced chlorophyll-binding protein}}
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[[Category:Bacterial proteins]]