The role of NOI-domain containing proteins in plant immune signaling

Abstract

Here we present an overview of our existing knowledge on the function of RIN4 as a regulator of plant defense and as a guardee of multiple plant R-proteins. Domain analysis of RIN4 reveals two NOI domains. The NOI domain was originally identified in a screen for nitrate induced genes. The domain is comprised of approximately 30 amino acids and contains 2 conserved motifs (PXFGXW and Y/FTXXF). The NOI gene family contains members exclusively from the plant lineage as far back as moss. In addition to the conserved NOI domain, members within the family also contain conserved C-terminal cysteine residue(s) which are sites for acylation and membrane tethering. Other than these two characteristic features, the sequence of the family of NOI-containing proteins is diverse and, with the exception of RIN4, their functions are not known. Recently published interactome data showing interactions between RIN4 and components of the exocyst complex prompt us to raise the hypothesis that RIN4 might be involved in defense associated vesicle trafficking.

Figure 1

The protein alignment composite of 51 NOI containing proteins from monocots, dicots and moss was generated using the Ensembl plant database (http://plants.ensembl.org/). The 51 NOI containing proteins were selected on the basis of their sequence homology to AtRIN4. Green bars show areas of amino acid alignment while the white areas are gaps in the alignment. Dark green bars indicate the consensus alignment in the collapsed tree. The regions corresponding to the C- and N- NOI domains are shown by red bars. The highest level of conservation in sequence is seen in the C-NOI domain, followed by the N-NOI domain and the region representing the conserved C-terminal cysteines.

Figure 2

(Left panel) A Phylogenetic tree of 51 NOI containing proteins from monocots, dicots and moss was generated using the Ensembl plant database (http://plants.ensembl.org/). The 51 NOI containing proteins were selected on the basis of their sequence homology to AtRIN4. Red nodes in the phylogenetic tree represent duplication events whereas the blue nodes are speciation events. (Right panel) Multiple alignments of the 51 NOI containing proteins shown on the left were generated using MUSCLE. The light green bars correspond to amino acids within the protein sequence, whereas the white spaces represent alignment gaps. The 3 intron junctions are shown as black lines (labeled) that run perpendicular to the alignment trace. The intron junction preceding the N-NOI is labeled as intron 1 whereas the intron junction preceding the C-NOI is labeled as intron 2. The position of the third intron varies between monocots and dicots. This junction in most monocots and moss is one amino acid removed from the conserved cysteines, whereas in most dicots it immediately precedes the C-terminal cysteine residues. The regions corresponding to the N-NOI and C-NOI are shown by red bars.