Abstract
BackgroundGuanylyl cyclases (GCs) catalyze the formation of the second messenger guanosine 3′,5′-cyclic monophosphate (cGMP) from guanosine 5′-triphosphate (GTP). Cyclic GMP has been implicated in an increasing number of plant processes, including responses to abiotic stresses such as dehydration and salt, as well as hormones.Principle FindingsHere we used a rational search strategy based on conserved and functionally assigned residues in the catalytic centre of annotated GCs to identify candidate GCs in Arabidopsis thaliana and show that one of the candidates is the brassinosteroid receptor AtBR1, a leucine rich repeat receptor like kinase. We have cloned and expressed a 114 amino acid recombinant protein (AtBR1-GC) that harbours the putative catalytic domain, and demonstrate that this molecule can convert GTP to cGMP in vitro.ConclusionsOur results suggest that AtBR1 may belong to a novel class of GCs that contains both a cytosolic kinase and GC domain, and thus have a domain organisation that is not dissimilar to that of atrial natriuretic peptide receptors, NPR1 and NPR2. The findings also suggest that cGMP may have a role as a second messenger in brassinosteroid signalling. In addition, it is conceivable that other proteins containing the extended GC search motif may also have catalytic activity, thus implying that a significant number of GCs, both in plants and animals, remain to be discovered, and this is in keeping with the fact that the single cellular green alga Chlamydomonas reinhardtii contains over 90 annotated putative CGs.
Highlights
Guanylyl cyclases (GCs) have been identified in many diverse prokaryotes and eukaryotes where they catalyse the formation of the second messenger guanosine 39,59-cyclic monophosphate from guanosine 59-triphosphate (GTP) [1]
Based on the binding of the ligand BR to the leucine rich repeat extracellular domain, Brassinosteroid Insensitive-1 (BRI1) has been identified as a BR receptor in Arabidopsis [29,30] and a critical signal component
BRI1 is ubiquitously expressed in Arabidopsis and potential BRI1 kinase substrates have been identified such as transthyretin-like protein which is phosphorylated in vitro by the kinase domain of BRI1 [31]
Summary
Guanylyl cyclases (GCs) have been identified in many diverse prokaryotes and eukaryotes where they catalyse the formation of the second messenger guanosine 39,59-cyclic monophosphate (cGMP) from guanosine 59-triphosphate (GTP) [1]. We show that a rationally designed search motif of the catalytic domain identifies several members of the family of leucine rich repeat receptor-like kinases (LRR RLKs) including an Arabidopsis thaliana brassinosteroid receptor (AtBRI1). A recombinant domain protein was made which tested positive for GC activity in vitro The implications of this finding for both the projected number of different classes of GCs and the role of cGMP in brassinosteroid signaling are discussed. We used a rational search strategy based on conserved and functionally assigned residues in the catalytic centre of annotated GCs to identify candidate GCs in Arabidopsis thaliana and show that one of the candidates is the brassinosteroid receptor AtBR1, a leucine rich repeat receptor like kinase. It is conceivable that other proteins containing the extended GC search motif may have catalytic activity, implying that a significant number of GCs, both in plants and animals, remain to be discovered, and this is in keeping with the fact that the single cellular green alga Chlamydomonas reinhardtii contains over 90 annotated putative CGs
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