One for all: the receptor-associated kinase BAK1

Abstract

The plant receptor kinase BAK1/SERK3 has been identified as a partner of ligand-binding leucine-rich repeat receptor kinases, in particular the brassinosteroid receptor BRI1 and the immune receptor FLS2. BAK1 positively regulates BRI1 receptor function via physical interaction and transphosphorylation. Since its first description in 2001, several independent groups have discovered BAK1/SERK3 as a component of diverse processes, including brassinosteroid signaling, light responses, cell death, and plant innate immunity. Here, we summarize current knowledge of the functional repertoire of BAK1 and discuss how its multiple functions could be integrated, how receptor complexes are potentially formed and how specificity might be determined.

Figure 1

The multiple functions of BAK1. (a) The phylogenetic tree of the SERK protein family indicates that SERK1 and 2 form a subgroup (blue) as well as SERK3 to 5 with SERK4 being the closest relative of BAK1 (red). The sequences were deduced from TAIR (www.arabidopsis.org) and the phylogenetic tree was generated with ClustalW. (b) Together with ligand binding receptors such as BRI1, FLS2, and additional unknown receptors (indicated by ???), BAK1 influences diverse processes such as somatic embryogenesis [1], tapetum formation [61,62], BR [2,3] and flagellin responses [7,8], cell death [5,6], light [4] and additional PAMP responses [7,8]. Some processes are synergistically influenced by more than one SERK protein, while some SERK proteins are involved in the regulation of multiple pathways [29] resulting in a complex functional network.

Figure 2

Model of the BRI1/BAK1 receptor complex activation. The initial stages of BR-dependent activation of BRI1 and BAK1 are shown. BRI1 exists as a homodimer even in the absence of the ligand [19]. Upon binding of BRs autophosphorylation commences from BRI1, resulting in a basal level of signaling, even in the complete absence of BAK1 or its orthologs SERK1 and BKK1 (both not shown here for simplicity) [24]. Release of BKI1 upon phosphorylation by BRI1 allows hetero-oligomerization with BAK1 [20]. Transphosphorylation of BAK1 by BRI1 then results in enhanced BRI1/BAK1 association [24,25]. (a)–(c) The reciprocal sequential phosphorylation events are shown for half of the heterotetrameric pair. Finally, transphosphorylation of BRI1 by BAK1 may result in full activation of the BR signaling complex [24]. The resulting tetrameric complexes are internalized into endosomal compartments [32].