InterPro: IPR002316 Prolyl-tRNA synthetase, class IIa, conserved region

The aminoacyl-tRNA synthetases (EC:6.1.1.) catalyse the attachment of an amino acid to its cognate transfer RNA molecule in a highly specific two-step reaction. These proteins differ widely in size and oligomeric state, and have limited sequence homology [1]. The 20 aminoacyl-tRNA synthetases are divided into two classes, I and II. Class I aminoacyl-tRNA synthetases contain a characteristic Rossman fold catalytic domain and are mostly monomeric [2]. Class II aminoacyl-tRNA synthetases share an anti-parallel beta-sheet fold flanked by alpha-helices [3], and are mostly dimeric or multimeric, containing at least three conserved regions [4, 5, 6]. However, tRNA binding involves an alpha-helical structure that is conserved between class I and class II synthetases. In reactions catalysed by the class I aminoacyl-tRNA synthetases, the aminoacyl group is coupled to the 2'-hydroxyl of the tRNA, while, in class II reactions, the 3'-hydroxyl site is preferred. The synthetases specific for arginine, cysteine, glutamic acid, glutamine, isoleucine, leucine, methionine, tyrosine, tryptophan and valine belong to class I synthetases; these synthetases are further divided into three subclasses, a, b and c, according to sequence homology. The synthetases specific for alanine, asparagine, aspartic acid, glycine, histidine, lysine, phenylalanine, proline, serine, and threonine belong to class-II synthetases [7].

Prolyl-tRNA synthetase (EC:6.1.1.15) exists in two forms, which are loosely related. The first form, is present in the majority of eubacteria species. The second one, present in some eubacteria, is essentially present in archaea and eukaryota. Prolyl-tRNA synthetase belongs to class IIa. The enzyme from Escherichia coli contains all three of the conserved consensus motifs characteristic of class II aminoacyl-tRNA synthetases [8]. The complex between Thermus thermophilus prolyl-tRNA synthetase (ProRSTT) and its cognate tRNA has been crystallized using two different isoacceptors of tRNA(Pro) [9].