15th International Conference on Arabidopsis Research - TAIR

T13-003
SH2 proteins in plants : The story is just beginning
Latha Kadalayil(1)
1-School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton,
SO16 7PX, England
The protein modules known as SH2 (Src-homology-2) domains are key
players in the signal transduction of animals. Two questions arise: Do such
modules exist in plants, and when did SH2 domains evolve? Here I show
that the Arabidopsis genome contains three strong candidates for plant
SH2 proteins (referred to as PASTA1, 2 and 3 : GI:25513455, At1g78540,
At1g17040 respectively) with homology to the SH2 domains and the adjacent
linker region of STAT proteins (Signal Transducer and Activator of Transcription).
The three characteristics features of a STAT protein sequence1, namely,
(i) the SH2 domain with a conserved arginine residue crucial for binding to a
phospho-tyrosine residue (ii) a tyrosine residue outside the C-terminus of the
SH2-domain for phosphorylation during signalling and (iii) a DNA-binding domain
are conserved in the PASTA3 protein. However, PASTA 1 and 2 proteins
lack a tyrosine in a similar position.
PASTA proteins are not homologous to STAT proteins outside the SH2 and
linker regions. The three PASTA proteins are 70 to 80 % identical to one
another.
Gene expression studies with PASTA2 reveal that it is expressed in all the
plant tissues tested. Preliminary indications are that plants homozygous for
PASTA2 do not have any obvious phenotype, most likely due to redundancies.
An antisense experiment with PASTA3 antisense DNA in PASTA2 mutant
background is being carried out presently.
STAT proteins need to be phosphorylated at tyrosine residues for their function.
Since no candidate gene for a plant protein tyrosine kinase has been
identified so far, it has been argued that plants are not likely to have STAT
proteins. Moreover, the current hypothesis is that tyrosine phosphorylation
is linked to metazoan evolution2. Based on the sequence homology of the
PASTA proteins to STAT proteins I propose the following. (1) The new proteins
PASTA1, 2 and 3 are Plant Analogues of the animal STAT proteins (hence
the name PASTA); (ii) the PASTA proteins function by a currently unknown
mechanism, which may not involve a tyrosine phosphorylation. These
observations suggest in turn that (iii) STAT-related proteins already existed in
the common ancestor of plants and animals, and (iv) the mechanism of STATmediated
signalling in plants and animals has subsequently diverged.
(1) Levy, D.E. and Darnell Jr, J.E. Nat. Rev. (2002) 3, 651 (2) Darnell Jr, J.E. Proc. Natl. Acad. Sci.
(1997) 94, 11767
T13 Others
T13-004
Autophosphorylation Activity of the Arabidopsis
Ethylene Receptor Multigene Family
Patricia Moussatche(1, 2), Harry J. Klee(1)
1-PMCB Program, Horticultural Sciences Department, University of Florida
2-Chemistry Department, University of Florida
Plant receptors for the gaseous hormone ethylene show sequence similarity
to bacterial two-component histidine kinases. These receptors are encoded
by a multigene family that can be divided into subfamilies 1 and 2. It has
been previously shown that a subfamily 1 Arabidopsis ethylene receptor,
ETR1, autophosphorylates in vitro on a conserved histidine residue (1).
However, sequence comparisons between the five ethylene receptor family
members suggest that subfamily 2 members do not have the motifs necessary
for histidine kinase activity. It has been reported that a tobacco subfamily
2 receptor, NTHK1, autophosphorylates on serines and threonines in vitro
(2). Here we show that all five Arabidopsis ethylene receptor proteins autophosphorylate
in vitro. We analyzed the nature of the phosphorylated amino
acids by acid/base stability and bi-dimensional thin layer electrophoresis, and
demonstrated that unlike ETR1 all other ethylene receptors autophosphorylate
predominantly on serine residues. ERS1, the only other subfamily 1
receptor, is able to phosphorylate on both histidine and serine residues in the
presence of Mn2+. However, this activity is lost when ERS1 is assayed in the
presence of both Mg2+ and Mn2+, suggesting that histidine autophosphorylation
may not occur in vivo unless a Mn2+ donor is present. Furthermore,
mutation of the histidine residue conserved in two-component systems does
not abolish serine autophosphorylation, discarding the possibility of a histidine
to serine phosphotransfer. Our biochemical observations complement the
recently published genetic data that histidine kinase activity is not necessary
for ethylene receptor function in plants and suggest that ethylene signal
transduction does not occur through a phosphorelay mechanism.
1) Gamble et al. (1998) Proc Natl Acad Sci USA. 95, 7825-7829.
2) Xie et al. (2003) Plant J. 33, 385-393.
15 th <strong>International</strong> <strong>Conference</strong> on Arabidopsis Research 2004 · Berlin