Arabidopsis kinase-associated protein phosphatase, KAPP was identified through interaction with the receptor-like protein kinase HAESA/RLK5 (Stone et al. 1994). Beside a catalytic PP2C part, KAPP protein contains the N-terminal membrane anchor and a kinase-interacting forkhead-associated domain (KI-FHA). The FHA domain is a phosphorylation-dependent protein-protein interaction domain with specificity for phosphor-threonine and possibly phosphor-serine. Solution dynamics and phospho-peptide binding effects have illustrated the KAPP KI-FHA structure (Ding et al. 2005, 2007; Lee et al. 2003). Through this domain, KAPP interacts with several phosphorylated RLKs, such as RLK4 (Braun et al. 1997), embryogenesis-specific AtSERK1 (Rienties et al. 2005; Shah et al. 2001, 2002) and the cell wall kinase WAK1 (Park et al. 2001). The components of the brassinosteroid receptor (BR), brassinosteroid-insensitive kinase 1 (BRI1), and BRI1-associated kinase 1 (BAK1) also interact with KAPP (Ding et al. 2007). KAPP interacts with and negatively regulates transmembrane receptor kinases CLAVATA1 (CLV1), which controls meristem development (Li et al. 1999; Stone et al. 1998; Williams et al. 1997), and FLAGELLIN SENSITIVE2 (FLS2), which is essential for flagellin perception in pathogen-associated innate immune response signalling (Gomez-Gomez et al. 2001). Interactions with diverse receptors imply KAPP control of multiple signalling pathways; however, no growth and developmental phenotypes were observed in T-DNA kapp-3 mutant (Ding et al. 2007). This is really surprising as KAPP has no close homologues, and at the same time suggests that there should be other phosphatases that act redundantly. A future task is to identify these proteins.

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