Abstract
BackgroundMany proteins with tandem repeats in their sequence have been described and classified according to the length of the repeats: I) Repeats of short oligopeptides (from 2 to 20 amino acids), including structural cell wall proteins and arabinogalactan proteins. II) Repeats that range in length from 20 to 40 residues, including proteins with a well-established three-dimensional structure often involved in mediating protein-protein interactions. (III) Longer repeats in the order of 100 amino acids that constitute structurally and functionally independent units. Here we analyse ShooT specific (ST) proteins, a family of proteins with tandem repeats of unknown function that were first found in Leguminosae, and their possible similarities to other proteins with tandem repeats.ResultsST protein sequences were only found in dicotyledonous plants, limited to several plant families, mainly the Fabaceae and the Asteraceae. ST mRNAs accumulate mainly in the roots and under biotic interactions. Most ST proteins have one or several Domain(s) of Unknown Function 2775 (DUF2775). All deduced ST proteins have a signal peptide, indicating that these proteins enter the secretory pathway, and the mature proteins have tandem repeat oligopeptides that share a hexapeptide (E/D)FEPRP followed by 4 partially conserved amino acids, which could determine a putative N-glycosylation signal, and a fully conserved tyrosine. In a phylogenetic tree, the sequences clade according to taxonomic group. A possible involvement in symbiosis and abiotic stress as well as in plant cell elongation is suggested, although different STs could play different roles in plant development.ConclusionsWe describe a new family of proteins called ST whose presence is limited to the plant kingdom, specifically to a few families of dicotyledonous plants. They present 20 to 40 amino acid tandem repeat sequences with different characteristics (signal peptide, DUF2775 domain, conservative repeat regions) from the described group of 20 to 40 amino acid tandem repeat proteins and also from known cell wall proteins with repeat sequences. Several putative roles in plant physiology can be inferred from the characteristics found.
Highlights
Many proteins with tandem repeats in their sequence have been described and classified according to the length of the repeats: I) Repeats of short oligopeptides, including structural cell wall proteins and arabinogalactan proteins
ShooT specific protein (ST) are specific to several plant families and form a multigenic family All ST sequences found in the different searches conducted in this work belonged to dicot plants and no similar sequences were found in kingdoms other than the plant kingdom
According to NCBI taxonomic classification, the phylogenetic distribution of all species having ST proteins as well as a few representative species without ST proteins (Figure 1) showed that no ST genes were found in green algae (Volvox carteri and Chlamydomonas reinhardtii), moss (Physcomitrella patens) and lycophyte (Selaginella moellendorffii) genomes
Summary
Many proteins with tandem repeats in their sequence have been described and classified according to the length of the repeats: I) Repeats of short oligopeptides (from 2 to 20 amino acids), including structural cell wall proteins and arabinogalactan proteins. There are three main classes of structural proteins in plant cell walls; the division is based on their amino acid composition and their repeating sequences: hydroxyproline-rich glycoproteins or extensins (HRGPs), proline-rich proteins (PRPs), and glycinerich proteins (GRPs) [4,5]. PRPs have been classified in five classes [8] on the basis of their motifs, domains and biochemical characteristics, and they were first identified as proteins that accumulate in the cell wall in response to physical damage [9]. Cell wall GRPs are often associated with the vascular system of plants, and their synthesis appears to be regulated during development [12]
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