Studies of the PfkB Family of Proteins in Arabidopsis thaliana
Authors: Riggs John W , Callis Judy 
Published: June 2021

The phosphofructokinase B (PkfB) family of carbohydrate kinases is a large group that includes members with specificity for a diverse array of carbohydrate substrates, including fructose, ribose, fructose‐6‐phosphate, tagatose‐6‐phosphate, adenosine, myo‐inositol, and others. Although there are 21 genes encoding PfkB family members in the higher plant Arabidopsis thaliana, only a few have been studied in detail. Two Arabidopsis proteins with fructokinase activity were identified and characterized via native PAGE and in‐gel assays, though the genes were not identified. Another class of PfkB proteins identified in Arabidopsis lacks demonstrated enzymatic activity, but is necessary for the accumulation of photosynthetic transcripts in chloroplasts. The fructokinase‐like proteins 1 and 2 (FLN1 and FLN2, respectively) and NARA5 all belong to this class, although the mechanism of how NARA5 exerts it's phenotype remains elusive. FLN1 and 2 have been identified in proteomics studies as members of a large protein complex that interacts with, and likely regulates, the prokaryotic‐type plastid RNA polymerase which specifically transcribes photosynthetic genes in the plastid.

We have found that FLN1 and FLN2 interact with each other in an ATP‐dependent manner both in vitro and in vivo. Phenotypic analysis suggests that the presence of both proteins is necessary for proper levels of transcription. Our data on the interaction between FLN1 and FLN2 suggests a mechanism where the FLN proteins act as a biochemical switch to activate transcription under conditions that favor their binding to each other.

We have also begun characterization of the remainder of the Arabidopsis PfkB family and have identified a group of seven highly similar Arabidopsis PfkB proteins. These seven proteins were expressed in E. coli and enzymatic assays performed. All are active as fructokinases with high specificity for fructose. T‐DNA mediated inactivation of five of the seven genes resulted in no overt phenotype which hints at possible redundancy between some of these gene products. Using GFP fusions, we have determined that all localize to the cytosol except one, which localizes to plastids. Higher order mutants and biochemical analysis, along with expression analysis will allow us to tease apart the differences and the importance of each of these family members.

We have characterized another PfkB protein that appears to be the sole Arabidopsis ribokinase (AtRBSK). Its biochemical properties and cofactor requirements are similar to that of known mammalian and prokaryotic RBSKs. T‐DNA insertion into AtRBSK results in the hyper‐accumulation of ribose in both the roots and leaves of 10 day‐old seedlings compared to wild type. Arabidopsis lacking AtRBSK grow poorly on media supplemented with ribose as the only sugar. We are currently dissecting the pathway that leads to the production of ribose in plants to put AtRBSK into a biological context.

Our studies of the PfkB family in Arabidopsis are yielding promising results and will continue to further our understanding of many fundamental processes in plant biology ranging from central metabolism to the regulation of transcription in chloroplasts.

Support or Funding Information

The authors gratefully acknowledge support from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, US. Department of Energy (contracts DE‐FG02–09ER16077 and DE‐SC0002175) to JC