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Received 8 April 2002; Accepted 4 September 2002 Introduction SnRK1 ( SNF1‐ Related Protein Kinase‐1) is a plant protein kinase with a catalytic domain similar to that of SNF1 ( Sucrose Non‐ fermenting‐1) of yeast and AMPK ( AMP‐activated protein kinase) of animals. The SNF1 family of protein kinases are a distinct group within the protein kinase superfamily, but are closely related to the calcium‐dependent protein kinase (CDPK) group, which includes the animal calmodulin‐dependent protein kinases and the plant calmodulin‐like domain protein kinases (). SNF1 () is activated in response to low cellular glucose levels and is required for the derepression of a battery of genes that are repressed by glucose (;; ). It also directly modulates the phosphorylation state of a number of metabolic enzymes, including acetyl‐CoA carboxylase () and glycogen synthase (), and is required for the arrest of growth and the cell cycle under conditions of glucose deprivation (). AMPK is activated by AMP (, 1989) and by phosphorylation by an upstream protein kinase (AMP‐activated protein kinase kinase (AMPKK)) (). Activation of AMPK by AMP is antagonized by high (mM) concentrations of ATP and a high AMP:ATP ratio is symptomatic of low cellular energy levels.

For this reason, AMPK has been likened to a cellular fuel gauge (). When activated, it acts to conserve ATP by phosphorylating and inactivating regulatory enzymes of ATP‐consuming, anabolic pathways such as acetyl‐CoA carboxylase (fatty acid synthesis) (, 1992) and HMG‐CoA reductase (sterol/isoprenoid synthesis) (; ). The SnRK gene family The first plant SnRK1 sequence to be reported was a cDNA isolated from a rye endosperm cDNA library (). It encoded a 57.7 kDa protein of 502 amino acid residues showing 48% amino acid sequence identity with SNF1 and AMPK, rising to 62–64% amino acid sequence identity in the kinase catalytic domain. SnRK1 genes have since been identified and characterized in many plant species (reviewed by ).

They are present in small to medium‐sized gene families, comprising, for example, three members in arabidopsis and 10–20 in barley. The SnRK1 gene family of cereals can be subdivided further into two groups, SnRK1a and SnRK1b, on the basis of amino acid sequence similarity and expression patterns ().

SnRK1a is expressed throughout the plant and is more closely related to SnRK1 from dicotyledonous plants. SnRK1b is expressed at the highest levels in the seed, although low levels of expression can be detected elsewhere, and is only present in monocotyledonous plants. The functional significance of this divergence of the gene family in cereals is not known.

Plants contain two other subfamilies of protein kinases, SnRK2 and SnRK3, containing catalytic domains with sequences that place them clearly within the SNF1 family. They have 42–45% amino acid sequence identity with SnRK1, SNF1 and AMPK in this region; they are, therefore, significantly less similar to SNF1 and AMPK than SnRK1 is. The SnRK2 and SnRK3 gene subfamilies appear to be unique to plants and are relatively large and diverse compared with SnRK1.

The SnRK2 subfamily includes PKABA1 from wheat, which is involved in mediating ABA‐induced changes in gene expression (; ). The SnRK3 gene family includes SOS2, an arabidopsis protein kinase involved in conferring salt tolerance (; ).

The completion of the arabidopsis genome sequencing project means that the full complement of the arabidopsis SnRK family can now be identified. These are shown as a dendrogram in Fig. SnRK1 activity The cloning of SnRK1 genes and cDNAs allowed antisera to be raised to heterologously expressed SnRK1 and to synthetic peptides based on the predicted SnRK1 peptide sequence. Use of these antibodies led to the confirmation that a plant protein kinase related to AMPK, that was being studied independently at the biochemical level, was, indeed, SnRK1 (; ). This made SnRK1 the first plant protein kinase for which biochemical and molecular biological studies came together. It had already been shown to phosphorylate the SAMS peptide (His Met Arg Ser Ala Met Ser Gly Leu His Leu Val Lys Arg Arg), a synthetic peptide based on the sequence around the primary phosphorylation site for AMPK on rat acetyl‐CoA carboxylase () and had been purified from cauliflower ().

Although it was not activated by AMP, it was found to have similar biochemical properties to those of mammalian AMPK in other respects (; ). It was shown to phosphorylate and inactivate a bacterially‐expressed arabidopsis HMG‐CoA reductase (). A recognition motif for SnRK1 was established using variant peptide substrates (). It comprises the phosphorylated serine (SnRK1 will phosphorylate threonine, but phosphorylates serine much more efficiently), hydrophobic residues at positions –5 and +4 relative to the serine, and at least one basic residue which could be at –3 or –4 (Fig. Volkswagen sharan rukovodstvo po ekspluatacii skachatj besplatno hd. The AMARA peptide (Ala Met Ala Arg Ala Ala Ser Ala Ala Ala Leu Ala Arg Arg Arg), in which the minimal recognition motif is retained, but other residues are alanine, apart from the basic C‐terminus which is not essential, appears to be a better substrate than the SAMS peptide ( b). More recently, basic residues at positions –6 and +5 have been shown to enhance activity and a proline residue at position –4 has been found to favour phosphorylation by SnRK1 relative to CDPKs (). The identification of peptide substrates for SnRK1 allowed SnRK1 activity to be measured using a convenient assay.