Agriculture Reference
In-Depth Information
8.6 Protein Phosphatases Involved in ABA Signaling:
PP2C, PP2A, and Others
8.6.1 Protein Phosphatase Family: PTP, PPP (PP1, PP2A,
PP2B), and PPM (PP2C, PDP)
Protein phosphatases can be divided into two major classes: protein tyrosine
phosphatases (PTPs) and protein serine/threonine phosphatases. Protein tyros-
ine phosphatases include PTPs and dual-specificity phosphatases (DSPTPs). The
Ser/Thr-specific phosphoprotein phosphatases (PPPs) execute the major phosphatase
activities in eukaryotes (Olsen et al. 2006 ). Based upon differential sensitivity to
small molecule inhibitors, the protein serine/threonine phosphatases are classified
into the PPP and PPM (for metal ion-dependent protein phosphatase) gene families.
The PPP family includes types 1 (PP1), 2A (PP2A), 2B (PP2B), whereas the PPM
family includes type 2C (PP2C) and pyruvate dehydrogenase phosphatase (PDP)
(Ingebritsen and Cohen 1983 ; Cohen 1997 ; Smith and Walker 1996 ; Luan 2003 ).
Among the PP2 members that vary in their subunit structure, divalent cation require-
ments, and substrate specificities, PP2A is a heterotrimer consisting of a catalytic
C subunit and two distinct regulatory A and B subunits, which does not require
divalent cations for its activity; PP2B is activated by Ca 2 + and is a heterodimer
of a catalytic A subunit and a regulatory B subunit; PP2B is an important Ca 2 + -
regulated PPP in other eukaryotes, but is absent in plants; PP2C exists as a mono-
mer and requires Mg 2 + for its activity (Cohen 1989 , 1997 ; Smith and Walker 1996 ;
Luan 2003 ). Three major classes of the plant protein phosphatases are homologs of
the mammalian type-1, -2A, and -2C protein serine/threonine phosphatases (Smith
and Walker 1996 ; Luan 2003 ; Schweighofer et al. 2004 ). The Arabidopsis genome
encodes more than 100 phosphatase catalytic subunit sequences (Arino et al. 1993 ;
Smith and Walker 1996 ; Schweighofer et al. 2004 ), suggesting that the protein phos-
phatases may function in multiple physiological processes in plants. Indeed, molecu-
lar genetic and biochemical studies reveal important roles of these enzymes in plant
cell events such as signal transduction, regulation of metabolism, cell cycle pro-
gression, hormonal regulation, and stress responses (Smith and Walker 1996 ; Luan
2003 ; Schweighofer et al. 2004 ; Farkas et al. 2007 ). Especially, some of the PP2C
members have been identified as critical components of ABA signaling, working
together with a class of the cytosolic PYR/PYL/RCAR receptors for ABA (reviewed
in Cutler et al. 2010 , and see Chap. 6 of this topic).
8.6.2 Group-A PP2Cs: Central, Upstream Repressors of
ABA Signaling
The Arabidopsis genome encodes 76 PP2C-type phosphatase candidates, which
are divided into ten groups (from A to J) (Kerk et al. 2002 ; Schweighofer et al.
2004 ). Group A contains nine members, six of which have been identified as
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