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Two novel types of hexokinases in the moss Physcomitrella patens.

Nilsson A, Olsson T, Ulfstedt M, Thelander M, Ronne H - BMC Plant Biol. (2011)

Bottom Line: However, we also found two new types of hexokinases with no obvious orthologs in vascular plants.Type C, encoded by a single gene, has neither transit peptide nor membrane anchor, and is found in the cytosol and in the nucleus.We conclude that the hexokinase gene family is more diverse in Physcomitrella, encoding two additional types of hexokinases that are absent in vascular plants.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Microbiology, Swedish University of Agricultural Sciences, Box 7025, SE-750 07 Uppsala, Sweden.

ABSTRACT

Background: Hexokinase catalyzes the phosphorylation of glucose and fructose, but it is also involved in sugar sensing in both fungi and plants. We have previously described two types of hexokinases in the moss Physcomitrella. Type A, exemplified by PpHxk1, the major hexokinase in Physcomitrella, is a soluble protein that localizes to the chloroplast stroma. Type B, exemplified by PpHxk2, has an N-terminal membrane anchor. Both types are found also in vascular plants, and localize to the chloroplast stroma and mitochondrial membranes, respectively.

Results: We have now characterized all 11 hexokinase encoding genes in Physcomitrella. Based on their N-terminal sequences and intracellular localizations, three of the encoded proteins are type A hexokinases and four are type B hexokinases. One of the type B hexokinases has a splice variant without a membrane anchor, that localizes to the cytosol and the nucleus. However, we also found two new types of hexokinases with no obvious orthologs in vascular plants. Type C, encoded by a single gene, has neither transit peptide nor membrane anchor, and is found in the cytosol and in the nucleus. Type D hexokinases, encoded by three genes, have membrane anchors and localize to mitochondrial membranes, but their sequences differ from those of the type B hexokinases. Interestingly, all moss hexokinases are more similar to each other in overall sequence than to hexokinases from other plants, even though characteristic sequence motifs such as the membrane anchor of the type B hexokinases are highly conserved between moss and vascular plants, indicating a common origin for hexokinases of the same type.

Conclusions: We conclude that the hexokinase gene family is more diverse in Physcomitrella, encoding two additional types of hexokinases that are absent in vascular plants. In particular, the presence of a cytosolic and nuclear hexokinase (type C) sets Physcomitrella apart from vascular plants, and instead resembles yeast, where all hexokinases localize to the cytosol. The fact that all moss hexokinases are more similar to each other than to hexokinases from vascular plants, even though both type A and type B hexokinases are present in all plants, further suggests that the hexokinase gene family in Physcomitrella has undergone concerted evolution.

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Overview of the hexokinase genes in Physcomitrella. Exons are shown as gray boxes and introns as solid black lines. The predicted exon/intron organization is based on existing cDNA sequences and, if cDNA sequences were missing or aberrantly spliced, on the known splice pattern of other plant hexokinase genes, provided that the consensus donor and acceptor splice sites are conserved. The predicted transit peptides in the type A hexokinases and the membrane anchors in the type B and D hexokinases are shown as small boxes under exon 1.
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Figure 1: Overview of the hexokinase genes in Physcomitrella. Exons are shown as gray boxes and introns as solid black lines. The predicted exon/intron organization is based on existing cDNA sequences and, if cDNA sequences were missing or aberrantly spliced, on the known splice pattern of other plant hexokinase genes, provided that the consensus donor and acceptor splice sites are conserved. The predicted transit peptides in the type A hexokinases and the membrane anchors in the type B and D hexokinases are shown as small boxes under exon 1.

Mentions: We have previously shown that the major hexokinase in Physcomitrella, PpHxk1, is responsible for most of the hexokinase activity in protonemal tissue extracts. Thus, 80% of the total glucose phosphorylating activity, including almost all of the activity in the chloroplast stroma, disappears when the PpHXK1 gene is disrupted [5]. However, the same experiment also showed that a minor glucose phosphorylating activity which is associated with chloroplast membranes is unaffected by the PpHXK1 disruption [5]. We therefore expected that other hexokinases would be responsible for the residual enzymatic activity that is independent of PpHxk1, and in particular for the activity that is associated with the membrane fraction. Consistent with this the genome sequence [31] revealed that there are no less than eleven hexokinase genes in Physcomitrella and we found that they can be grouped into four different types that show some variation in their exon-intron organization (Figure 1). This exceeds the number of genes in both Arabidopsis (six) and rice (ten). It has previously been noted that metabolic enzymes are overrepresented in Physcomitrella, possibly reflecting a more diverse metabolism in mosses than in seed plants [41].


Two novel types of hexokinases in the moss Physcomitrella patens.

Nilsson A, Olsson T, Ulfstedt M, Thelander M, Ronne H - BMC Plant Biol. (2011)

Overview of the hexokinase genes in Physcomitrella. Exons are shown as gray boxes and introns as solid black lines. The predicted exon/intron organization is based on existing cDNA sequences and, if cDNA sequences were missing or aberrantly spliced, on the known splice pattern of other plant hexokinase genes, provided that the consensus donor and acceptor splice sites are conserved. The predicted transit peptides in the type A hexokinases and the membrane anchors in the type B and D hexokinases are shown as small boxes under exon 1.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3045890&req=5

Figure 1: Overview of the hexokinase genes in Physcomitrella. Exons are shown as gray boxes and introns as solid black lines. The predicted exon/intron organization is based on existing cDNA sequences and, if cDNA sequences were missing or aberrantly spliced, on the known splice pattern of other plant hexokinase genes, provided that the consensus donor and acceptor splice sites are conserved. The predicted transit peptides in the type A hexokinases and the membrane anchors in the type B and D hexokinases are shown as small boxes under exon 1.
Mentions: We have previously shown that the major hexokinase in Physcomitrella, PpHxk1, is responsible for most of the hexokinase activity in protonemal tissue extracts. Thus, 80% of the total glucose phosphorylating activity, including almost all of the activity in the chloroplast stroma, disappears when the PpHXK1 gene is disrupted [5]. However, the same experiment also showed that a minor glucose phosphorylating activity which is associated with chloroplast membranes is unaffected by the PpHXK1 disruption [5]. We therefore expected that other hexokinases would be responsible for the residual enzymatic activity that is independent of PpHxk1, and in particular for the activity that is associated with the membrane fraction. Consistent with this the genome sequence [31] revealed that there are no less than eleven hexokinase genes in Physcomitrella and we found that they can be grouped into four different types that show some variation in their exon-intron organization (Figure 1). This exceeds the number of genes in both Arabidopsis (six) and rice (ten). It has previously been noted that metabolic enzymes are overrepresented in Physcomitrella, possibly reflecting a more diverse metabolism in mosses than in seed plants [41].

Bottom Line: However, we also found two new types of hexokinases with no obvious orthologs in vascular plants.Type C, encoded by a single gene, has neither transit peptide nor membrane anchor, and is found in the cytosol and in the nucleus.We conclude that the hexokinase gene family is more diverse in Physcomitrella, encoding two additional types of hexokinases that are absent in vascular plants.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Microbiology, Swedish University of Agricultural Sciences, Box 7025, SE-750 07 Uppsala, Sweden.

ABSTRACT

Background: Hexokinase catalyzes the phosphorylation of glucose and fructose, but it is also involved in sugar sensing in both fungi and plants. We have previously described two types of hexokinases in the moss Physcomitrella. Type A, exemplified by PpHxk1, the major hexokinase in Physcomitrella, is a soluble protein that localizes to the chloroplast stroma. Type B, exemplified by PpHxk2, has an N-terminal membrane anchor. Both types are found also in vascular plants, and localize to the chloroplast stroma and mitochondrial membranes, respectively.

Results: We have now characterized all 11 hexokinase encoding genes in Physcomitrella. Based on their N-terminal sequences and intracellular localizations, three of the encoded proteins are type A hexokinases and four are type B hexokinases. One of the type B hexokinases has a splice variant without a membrane anchor, that localizes to the cytosol and the nucleus. However, we also found two new types of hexokinases with no obvious orthologs in vascular plants. Type C, encoded by a single gene, has neither transit peptide nor membrane anchor, and is found in the cytosol and in the nucleus. Type D hexokinases, encoded by three genes, have membrane anchors and localize to mitochondrial membranes, but their sequences differ from those of the type B hexokinases. Interestingly, all moss hexokinases are more similar to each other in overall sequence than to hexokinases from other plants, even though characteristic sequence motifs such as the membrane anchor of the type B hexokinases are highly conserved between moss and vascular plants, indicating a common origin for hexokinases of the same type.

Conclusions: We conclude that the hexokinase gene family is more diverse in Physcomitrella, encoding two additional types of hexokinases that are absent in vascular plants. In particular, the presence of a cytosolic and nuclear hexokinase (type C) sets Physcomitrella apart from vascular plants, and instead resembles yeast, where all hexokinases localize to the cytosol. The fact that all moss hexokinases are more similar to each other than to hexokinases from vascular plants, even though both type A and type B hexokinases are present in all plants, further suggests that the hexokinase gene family in Physcomitrella has undergone concerted evolution.

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