ST proteins, a new family of plant tandem repeat proteins with a DUF2775 domain mainly found in Fabaceae and Asteraceae

ST 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.We 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.One characteristic of many proteins is the presence of tandem repeats in their sequence. It has been estimated that approximately 14% of all known proteins contain significant internal repeats, most of them in eukaryotic organisms [1].Different categories of tandem repeats can be defined [2]. (I) Repeats of short oligopeptides (from 2 to 20 amino acids) that are unlikely to form structural or functional units by themselves and become a single functional entity when interactions among repeats take place. (II) Repeats that range in length from 20 to 40 residues, which form structurally indepe