Proteins Function with Partners

Most proteins do not function alone, but rather display their activity in protein complexes. Thus, each protein at some point or another undergoes protein-protein interactions with one or more other proteins. For example, membrane receptors undergo interactions with downstream signaling pro-

teins. This has been especially well studied in the plant sensor histidine kinase receptors, for which the specificity and cross-talk between signaling pathways is defined through interaction specificity and generality. For example, the sensor histidine kinase CRE1 can interact with the phospho-transfer proteins AHP1 and AHP2, while the histidine kinase CKI1 specifically interacts only with AHP2, and the ethylene receptor ETR1 again can interact with AHP1 and AHP2 (Grefen and Harter 2004). Similarly, other receptor kinases, such as the LRR-receptor kinase BRI1, are starting to move into our focus as a model system for protein-protein interactions and phosphorylation in a plant signaling pathway (Ehsan et al. 2005; Wang et al. 2005).

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