Structural basis for a novel type of cytokinin-activating protein

The Lonely Guy (LOG) protein has been identified as a crucial enzyme involved in the production of cytokinins, which are important phytohormones, in plants and plant-interacting organisms. However, C. glutamicum has an isoform (Cg1261) of LOG that contains an extended N-terminal region compared to those of known LOGs, and this type of isoforms are also found in a variety of organisms. Nevertheless, these proteins are considered as lysine decarboxylases, without their functional characterization. To investigate the function of Cg1261, we determined its crystal structure at a resolution of 1.95 Å. Unlike known dimeric LOGs, Cg1261 was found to form a hexamer. The overall shape of the hexamer resembles a trillium flower, in which a twisted dimer constitutes each petal. The dimeric petal is well superposed with known LOG dimers, and its active site conformation is similar to those of LOG dimers, suggesting that the hexameric LOG-like protein also acts as a LOG. Biochemical and in vivo cytokinin production studies on Cg1261 confirms that Cg1261 functions as a cytokinin-activating protein. Phylogenetic tree analysis using 123 LOG-like proteins suggest that the LOG-like proteins can be categorized to the dimeric type-I LOG and the hexameric type-II LOG.