Members of this family are predominantly found in prokaryotic prolyl-tRNA synthetase. They contain a zinc binding site, and adopt a structure consisting of alpha helices and antiparallel beta sheets arranged in 2 layers, in a beta-alpha-beta-alpha-beta motif .
Proline tRNA ligase (also known as Prolyl tRNA synthetase) (EC) 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. Proline-tRNA ligase belongs to class IIa.