Plants respond to pathogen infection or environmental stress by increasing the expression of a number of genes that encode pathogenesis-related proteins. Up to date, PR proteins have been classified into 14 families based on their biological role and/or physicochemical properties. The biological functions of most classes of the defense proteins have been recognized. The role of some PR proteins, including PR-10, in defense response still remains to be elucidated. Amino acid sequence alignment as well as phylogenetic analysis allows distinguishing three major groups of that protein class: (i) Intracellular pathogenesis-related (IPR). Many IPR proteins have allergenic properties and can be found in pollen grains, fruits and vegetables. Almost all of these proteins cause IgE-mediated type I allergy. On the basis of weak sequence homology, ranging from 17 to 20 %, the presence of characteristic sequence motifs and secondary structure predictions, two additional distinct groups of proteins have been tentatively included in the PR-10 class: (ii) Major latex proteins (MLP) present in the latex of some plants (opium poppy, bell pepper) and (iii) Cytokinin-specific binding proteins (CSBP), for which the ability of plant hormones binding have been shown. The phylogenetic tree clearly distinguishes the three groups. The presence of sequences from the same species in different branches can be explained by numerous gene duplications which occurred in the past and led to creation of numerous copies of the same gene in a common ancestor. The oldest of these duplications took place before the separation of monocots and dicots and resulted in arising of the three major groups described above. Subsequent duplications occurred before the emerging of present species. It is clearly visible in legume subtree where, for example, the sequences of yellow lupine IPR proteins group in different branches resulting in the presence of different PR-10 protein subclasses within one species. Within subclasses, most likely due to concerted evolution, a large number of very similar genes (97-99 % identity) coexist. The consequence of the numerous gene duplications is coding of PR-10 proteins by multigene families. All studied PR-10 proteins, including CSBP proteins represent the same canonical model of the crystal structure. It is possible that the CSBP proteins have evolved from the PR-10 group to specifically bind only one group of ligands, cytokinins. Since they are cytosolic proteins, a protective or transport function within the plant cell could be proposed. The low level of sequence conservation between the CSBP and PR-10 groups suggests an early divergence or fast mutation rate. In addition to mutations affecting the volume and chemical character of the ligand binding cavity, there are numerous other changes. Nevertheless, the evolutionary pressure has left the general protein fold intact, suggesting its importance and confirming the ligand-binding role for PR-10 proteins.

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