Protein Kinases and Phosphatases Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 156

phosphatase 2A is a holoenzyme composed of a scaffolding subunit A, a regula-
tory subunit B, and a catalytic subunit C. The subunits A and C directly interact to
form the AC core enzyme, which binds subunit B to form a variety of heterotrim-
eric complexes (Cohen 1989 , 1997 ; Mayer-Jaekel and Hemmings 1994 ; Smith and
Walker 1996 ; Luan 2003 ). The regulatory subunit B determines the substrate spec-
ificity, subcellular localization, and catalytic activity of the PP2As (Virshup 2000 ;
Janssens and Goris 2001 ; Luan 2003 ). For PP2A family proteins, the Arabidopsis
genome encodes five catalytic C subunits (Arino et al. 1993 ; Smith and Walker
1996 ; Kerk et al. 2002 ; Schweighofer et al. 2004 ), three scaffolding A subunits
(Slabas et al. 1994 ; Garbers et al. 1996 ), and 17 regulatory B subunits (Haynes
et al. 1999 ; Terol et al. 2002 ; Farkas et al. 2007 ). The multiple isoforms may com-
bine to form a wide variety of functional complexes, theoretically accounting for
255 different heterotrimer combinations that could eventually perform distinct
functions in the regulation of different processes or redundant functions in specific
processes (Janssens et al. 2005 ; Farkas et al. 2007 ).
Earlier pharmacological studies suggested that protein phosphatases PP1 and
PP2A might function as positive or negative regulators of ABA signaling (Schmidt
et al. 1995 ; Esser et al. 1997 ; Hey et al. 1997 ; Wu et al. 1997 ). Further genetic
approaches in Arabidopsis allowed to identify a mutant rcn1 with reduced levels
of PP2A activity, harboring a recessive disruption in the RCN1 gene encoding a
guard cell-expressed PP2A scaffolding A subunit (PP2A-A1); the mutant shows
ABA insensitivity in ABA inhibition of seed germination, ABA-induced stomatal
closing, and ABA activation of slow anion channels, suggesting that the RCN1 is a
positive regulator of ABA signal transduction in Arabidopsis (Kwak, et al. 2002 ).
However, RCN1 is also involved in auxin transport and ethylene responses whose
mutants therefore exhibit pleiotropic phenotypes resulting from a complicated
cross talk among the three hormones (Rashotte et al. 2001 ; Larsen and Cancel
2003 ). A T-DNA insertional mutant pp2ac - 2 with loss of function of a PP2A
catalytic subunit (PP2Ac) was identified more recently, in which the total PP2A
activity is reduced (Pernas et al. 2007 ). The pp2ac - 2 mutant shows ABA hypersen-
sitivity in seed germination, dormancy, seedling growth, and ABA-dependent gene
expression, while the PP2Ac - 2 overexpressing lines show ABA-insensitive pheno-
types (Pernas et al. 2007 ). These results reveal that PP2Ac-2 is a negative regula-
tor of ABA signal transduction in Arabidopsis . Given that PP2Ac-2 is a catalytic
subunit with a specific role in ABA signaling, the identification of PP2Ac-2 as a
negative regulator of ABA responses suggests that PP2As are likely to function
as a negative, but not positive, component in the ABA-signaling pathway (Pernas
et al. 2007 ).
Recently, a study showed that FyPP1 and FyPP3 (for Phytochrome-associated
serine/threonine protein phosphatase1/3), two homologous catalytic subunits of
PP6, are negatively involved in ABA signaling in Arabidopsis (Dai et al. 2013 ).
It is noteworthy that there are high sequence similarities among the C subunits of
PP6 and PP2A phosphatases, while the PP6 activity requires Zn 2 + , but PP2A does
not require divalent cations for its activity as mentioned above (Cohen 1989 , 1997 ;
Smith and Walker 1996 ; Luan 2003 ; Wang et al. 2007 ; Dai et al. 2012 ). Genetic