WRKY proteins are a large group of transcription factors restricted to the plant kingdom. In Arabidopsis thaliana, the gene family consists of 74 members. Here, we analyzed the expression of all 13 members of one main WRKY subgroup and found that the majority are responsive both to pathogen infection and to salicylic acid. Temporal expression studies during compatible, incompatible, and nonhost interactions and employing plant defense-signaling mutants allowed us to define four distinct WRKY subsets responding to different signaling queues along defense pathways. These subsets did not reflect phylogenetic relationships. Promoter studies of one member, AtWRKY54, using a reporter gene construct in transgenic Arabidopsis plants, revealed that regulatory regions mediating pathogen and SA inducibility are clearly separable. In an AtWRKY54 knockout line, resistance to Peronospora parasitica was not compromised, but the transient expression kinetics of several WRKY genes was affected, suggesting both the existence of functional redundancy and intense cross-talk between signaling networks.

IntroductionClathrin-mediated endocytosis (CME) serves to take up nutrients from the extracellular environment into a eukaryotic cell and is also required to clear the plasma membrane (PM) of signalling receptors (reviewed by Mellman, 1996;Cavalli et al., 2001). CME is initiated at specific sites of the PM where the recruitment of adaptor 2 (AP2) complexes, clathrin and endocytic network proteins takes place. This is followed by the invagination of clathrin-coated pits (CCP) and finally culminates in the release of coated endocytic vesicles into the cytoplasm. After shedding their coat, the naked endocytic vesicles are then able to fuse with an early endocytic compartment, thereby releasing their cargo into the endocytic pathway (reviewed by Gruenberg, 2001).Dissection of each single step during the process of CME has revealed that distinct subsets of cytosolic endocytic network proteins are required for efficient clathrin and cargo recruitment (Brodsky et al., 2001;Higgins and McMahon, 2002). Thus, the various components of the clathrin machinery can be divided into two groups. The first group comprises the true coat components: clathrin, the AP2 complex, AP180, auxilin and HIP1/HIP1R (Kirchhausen, 1999;Kirchhausen, 2000a; Engqvist-Goldstein et al., 2001;Scheele et al., 2001), whereas the second group consists of cytosolic network proteins (e.g. eps15, amphiphysin, epsin and dynamin), which associate only transiently with the coat proteins, but also with each other in order to perform their functions (Kichhausen, 2000b; Slepnev and DeCamilli, 2000). A cross-section through a clathrin-coated vesicle (CCV) therefore reveals a threelayered structure: the outer coat layer of clathrin triskelia is connected via the middle layer of AP2 complexes to the inner layer of integral proteins of the vesicle membrane (Pearse et al., 2000).Of the four mammalian AP complexes the AP2 complex, which is exclusively associated with endocytic CCV, is the best characterized (Brett et al., 2002;Collins et al., 2002). Like the others, it is a heterotetrameric complex consisting of two large (αA-or αC-, β2-adaptin, ~100 kDa), one medium (µ2-adaptin, 50 kDa) and one small subunit (σ2-adaptin, ~20 kDa) (Kirchhausen, 1999). With the exception of the small subunit, all the others have well-described functions assigned to their specific domains. Accordingly, the C-terminal ear domain of αC-adaptin serves as a binding platform for several network proteins, which in turn contain specific αC-ear-domain binding motifs (DPF, DPW or FXDXF) in a variable number and composition (Owen et al., 1999;Brett et al., 2002). Likewise, the monomeric neuronal coat protein AP180 also contains within its C-terminal disordered region several consensus 2051 Clathrin-mediated endocytosis is a well-studied uptake mechanism for nutrients and signalling receptors in mammalian cells that depends on the coordinated interaction of coat proteins and endocytic network proteins to perform the internalization. In this process AP180 promotes the assembly of clathrin triskel...