Molecular insights into the function of ankyrin proteins in plants

Ankyrin (ANK) repeat domain-containing proteins comprise one of the largest known protein superfamilies in all species including plants. Recently, several genomeanalysis studies have provided valuable information on the structure of ANK proteins in plants. Among the 13 subgroups based on the presence of various additional domains in addition to the ANK domain, the E3 ubiquitin ligase activity and transcriptional regulation functions of ANK-RF and ANK-ZF subgroup members, respectively, are relatively well understood. NPR1 (nonexpressor of pathogenesis-related1), a key regulator of systemic acquired resistance in Arabidopsis, is a noteworthy member of the ANK-BTB subgroup; however, ANK-M and ANK-TM, the two main subgroups, have been less functionally characterized. With the ability to mediate protein-protein interactions, the majority of plant ANK proteins play crucial roles in defense responses and, on occasion, functions in growth and development. In this review, we summarize on the current knowledge of plant ANK superfamily members and focus on ANK proteins involved in defense responses. In addition, we provide a valuable framework for the future functional characterization of ANK genes with current unknown function in rice, a model crop species.