User:Alice Harmon/Sandbox 4
SnRK2.6/OST1/SRK2ESnRK2.6/OST1/SRK2E
Role in Abscicic Acid signalingRole in Abscicic Acid signaling
SnRK2.6/OST1/SRK2E is a triply-named protein kinase from Arabidopsis, which, along with its homologs, are activated by the abscisic acid (ABA) response pathway in plants. As shown in the figure, in unstimulated cells SnRK2.6/OST1/SRK2E (K in the figure) and a protein phosphatase 2C (ABI1 or its homologs; P in the figure) are in a complex (K-P) in which the kinase is dephosphorylated and inactive. When ABA levels in the cytosol rise, ABA binds to an ABA receptor (PYR1/PYL/RCAR; R in the figure). The activated receptor steals the protein phosphatase from its complex SnRK2.6/OST1/SRK2E and forms its own complex with the phosphatase. SnRK2.6/OST1/SRK2E is now free to be activated by autophosphorylation or phoshorylation by another protein kinase. Activation of this pathway by ABA leads to phosphorylation by SnRK2.6/OST1/SRK2E of: 1) ion channels SLAC1[1] and KAT1[2] in guard cells and stomatal closure; 2) transcription factor ABI5[3] in seeds/seedlings and dormancy/growth arrest; or 3) phosphorylation of transcription factor AREB/ABF [4][5] in vegetative tissue and stress tolerance and growth regulation.
PYR2-ABA 3kdi pYR2 3kdh PYR2-ABI2 3ujl
Kinase names and family membersKinase names and family members
Two of the SnRK2.6/OST1/SRK2E's three names originated from its membership in subclass III of the SnRK2 family of protein kinases. It was named SnRK2.6 by Hrabak et al **REf** and SRK2E by Umezawa et al[6]. SnRK2 stands for SNF1-related kinase group 2, which in Arabidopsis has 10 members. SNRK2s are members of the calmodulin-dependent protein kinase clade of protein kinases. The third name OST1 (open stomata 1)[7] is descriptive of the phenotype of plants bearing a gene mutation that produced an inactive protein kinase.
Two other family members in Arabidopsis, SNRK2.2/SRK2D and SnRK2.3/SRK2I, are activated by the ABA pathway in the same manner as SnRK2.6. Each of these kinases interacts with a member of clade A of the protein phosphatase 2C family - ABI1, HAB1 or HAB2. In rice homologs of these protein kinases are named SAPK8, SAPK9 and SAPK10.
Kinase structure and regulationKinase structure and regulation
SnRK2.6/OST1/SRK2E has an amino acid sequence typical of members of the SNRK2 family: an amino terminal Eukaryotic Protein Kinase Catalytic Domain and a C-terminal sequence that contains the SNRK2 box, which is unique to the family and required for activity[8] The C-terminus also contains a sequence called the ABA box, which is found only in the family members that are responsive to ABA[9]. The latter sequence is required for binding to PP2C[10], but is not seen in the crystal structure (why??) (see below).
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The structure of the catalytic domain is typical of protein kinases (see Eukaryotic Protein Kinase Catalytic Domain) except for an additional alpha helix in the small lobe that is formed by SNRK2 box sequence. This helix parallels subdomain III and resembles **blah blah** kinase structures.
Activation loop (with gap) in blue
Catalytic loop in orchid
C-helix in yellow
K in chartreuse
Snrk2 box (unique to Snrk's) in turquoise.
C-terminal acidic domain missing.
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The SnRK2.6–HAB1 complex was constructed as a fusion protein with a H6-tag (MAHHHHHHA) at the N-terminus of SnRK2.6 (residues 11–362) fused to HAB1(172–511) with a GSGSAGSAAGS linker.
D296A and E297A surface entropy reduction mutation sites
ABA box is not seen in the crystal structure.
Clearest picture of Pyr1 gate - latch is in Annu Reviews article
kinase activation loop in blue
phosphatase W385 in brown
unused:
SNRK2 structuresSNRK2 structures
3uc3 Arabidopsis thaliana SNRK2.3 + Co2+
3zut AtSNRK2.6 (D160A mutant)+ ANP
3zuu AtSNRK2.6 (D160A, S175D mutant) + gold
3uc4 apoAtSNRK2.6 (D59A, E60A mutant)
3udb apoAtSNRK2.6 (C131A, C157A, C159A, S7A, s29A, s43A, S166A, T175A)
complex with a protein phosphatase 2C
3ujg AtSNRK2.6 (D296A) + HAB1 + Mg2+
PP2C structuresPP2C structures
1a6q – hPP2C
2iq1 – hPP2C κ
2p8e – hPP2C β
2cm1 – PP2C + Mn – Micobacterium tuberculosis
3d8k – PP2C – Toxoplasma gondii
3jrq, 3kdj, 3nmn – AtPP2C + Pyl1 – Arabidopsis thaliana
3nmt, 3kb3, 3nmv, 3ujl – AtPP2C + Pyl2
4ds8 – AtPP2C + Pyl3 + Mn
3rt0 – AtPP2C (mutant) + Pyl10
3qn1, 3zvu – AtPP2C + Pyr1
3ujg – AtPP2C + SNRK2
3ujk – AtPP2C
ReferencesReferences
- ↑ Geiger D, Scherzer S, Mumm P, Stange A, Marten I, Bauer H, Ache P, Matschi S, Liese A, Al-Rasheid KA, Romeis T, Hedrich R. Activity of guard cell anion channel SLAC1 is controlled by drought-stress signaling kinase-phosphatase pair. Proc Natl Acad Sci U S A. 2009 Dec 15;106(50):21425-30. doi:, 10.1073/pnas.0912021106. Epub 2009 Dec 2. PMID:19955405 doi:10.1073/pnas.0912021106
- ↑ Sato A, Sato Y, Fukao Y, Fujiwara M, Umezawa T, Shinozaki K, Hibi T, Taniguchi M, Miyake H, Goto DB, Uozumi N. Threonine at position 306 of the KAT1 potassium channel is essential for channel activity and is a target site for ABA-activated SnRK2/OST1/SnRK2.6 protein kinase. Biochem J. 2009 Dec 10;424(3):439-48. doi: 10.1042/BJ20091221. PMID:19785574 doi:10.1042/BJ20091221
- ↑ Nakashima K, Fujita Y, Kanamori N, Katagiri T, Umezawa T, Kidokoro S, Maruyama K, Yoshida T, Ishiyama K, Kobayashi M, Shinozaki K, Yamaguchi-Shinozaki K. Three Arabidopsis SnRK2 protein kinases, SRK2D/SnRK2.2, SRK2E/SnRK2.6/OST1 and SRK2I/SnRK2.3, involved in ABA signaling are essential for the control of seed development and dormancy. Plant Cell Physiol. 2009 Jul;50(7):1345-63. doi: 10.1093/pcp/pcp083. Epub 2009, Jun 18. PMID:19541597 doi:10.1093/pcp/pcp083
- ↑ Furihata T, Maruyama K, Fujita Y, Umezawa T, Yoshida R, Shinozaki K, Yamaguchi-Shinozaki K. Abscisic acid-dependent multisite phosphorylation regulates the activity of a transcription activator AREB1. Proc Natl Acad Sci U S A. 2006 Feb 7;103(6):1988-93. Epub 2006 Jan 30. PMID:16446457 doi:10.1073/pnas.0505667103
- ↑ Fujii H, Verslues PE, Zhu JK. Identification of two protein kinases required for abscisic acid regulation of seed germination, root growth, and gene expression in Arabidopsis. Plant Cell. 2007 Feb;19(2):485-94. Epub 2007 Feb 16. PMID:17307925 doi:tpc.106.048538
- ↑ Umezawa T, Sugiyama N, Mizoguchi M, Hayashi S, Myouga F, Yamaguchi-Shinozaki K, Ishihama Y, Hirayama T, Shinozaki K. Type 2C protein phosphatases directly regulate abscisic acid-activated protein kinases in Arabidopsis. Proc Natl Acad Sci U S A. 2009 Oct 13;106(41):17588-93. doi:, 10.1073/pnas.0907095106. Epub 2009 Sep 29. PMID:19805022 doi:10.1073/pnas.0907095106
- ↑ 12468729
- ↑ Belin C, de Franco PO, Bourbousse C, Chaignepain S, Schmitter JM, Vavasseur A, Giraudat J, Barbier-Brygoo H, Thomine S. Identification of features regulating OST1 kinase activity and OST1 function in guard cells. Plant Physiol. 2006 Aug;141(4):1316-27. Epub 2006 Jun 9. PMID:16766677 doi:pp.106.079327
- ↑ Belin C, de Franco PO, Bourbousse C, Chaignepain S, Schmitter JM, Vavasseur A, Giraudat J, Barbier-Brygoo H, Thomine S. Identification of features regulating OST1 kinase activity and OST1 function in guard cells. Plant Physiol. 2006 Aug;141(4):1316-27. Epub 2006 Jun 9. PMID:16766677 doi:pp.106.079327
- ↑ Belin C, de Franco PO, Bourbousse C, Chaignepain S, Schmitter JM, Vavasseur A, Giraudat J, Barbier-Brygoo H, Thomine S. Identification of features regulating OST1 kinase activity and OST1 function in guard cells. Plant Physiol. 2006 Aug;141(4):1316-27. Epub 2006 Jun 9. PMID:16766677 doi:pp.106.079327