Legumains - a family of asparagine-specific cysteine endopeptidases involved in propolypeptide processing and protein breakdown in plants

Legumains are a recently discovered family of plant and animal cysteine endopeptidases with a cleavage specificity for Asn in the P1 position of peptide bonds. Asp-flanked peptide bonds also are cleaved, but with a much lower efficiency. Legumains evolved from GPI transamidase-like progenitors. Sequence analysis revealed three major groups of plant legumains corresponding to differences in the developmental and organ-specific gene expression. With the exception of a single cell wall specific representative, all legumains occur in the vacuolar compartment. Legumains are either involved in protein degradation or play a role in the processing of precursor proteins by Asn/Asp-specific limited proteolysis. Which function legumains perform depends on the conformational state of the substrate protein. A legumain acts as a vacuolar processing enzyme when it only has access to the regular processing sites of a precursor polypeptide, but it acts as a degradative enzyme when an altered conformation opens the substrate for unlimited proteolysis. The specificity of these interactions seems to be the result of a co-evolution of enzyme and substrate. The double function of legumains is particularly evident in the events of deposition and mobilisation of storage globulins during seed maturation and germination/seedling growth and in senescing and dying cells.