Plant HMG proteins bearing the AT-hook motif

Abstract

The high mobility group (HMG) proteins are non-histone components of chromatin that regulate gene expression via interactions with chromatin and transactivating protein factors. Plants and animals possess a family of HMG proteins that are similar on the basis of a shared motif known as the AT-hook, a domain that preferentially recognizes and binds to DNA with certain structural features, such as those imparted by AT-rich DNA. The animal HMG proteins with AT-hooks, collectively known as the HMGA family, have been studied extensively. As a consequence, much of the information available on the function of the animal HMGA family has been inferred to the plant HMG-I/Y family of AT-hook proteins. While both animal HMGA proteins and plant HMG-I/Y proteins have AT-hook motifs, the overall structure of these proteins is quite different. Nevertheless, both plant and animal AT-hook proteins bind AT-rich tracts of DNA in the minor groove, induce DNA bending, and function in the regulation of gene expression. Based on detailed studies of HMGA-mediated regulation of mammalian genes, analogous mechanisms of HMG-I/Y-mediated gene regulation in plants have been proposed. This review focuses primarily on the expression of the AT-hook HMG proteins and the proposed mechanisms by which these proteins regulate gene expression in plants and animals. An understanding of the variety of interactions of these proteins with chromatin in all eukaryotes is likely to provide insight into the regulatory properties of plant HMG-I/Y proteins.