The Medicago truncatula small protein proteome and peptidome.

The small protein and native peptide component of plant tissues is a neglected area of proteomic studies. We have used fractionation techniques for denatured and nondenatured protein preparations combined with 2-D LC tandem mass spectrometry to examine the sequences of small proteins and peptides in four tissues of the model legume, Medicago truncatula: the root tip and root of germinating seedlings, nitrogen fixing nodules, and young leaves. The isolation and fractionation strategies successfully enriched the small protein and native peptide content of the samples. Eighty-one small M. truncatula proteins and native peptides were identified. Most samples were dominated by ribosomal and histone proteins, and leaf samples possessed photosynthesis-related proteins. Secreted proteins such as lipid transfer proteins were common to several tissues. Twenty-four hours after germination, the roots and root tip tissues possessed several "seed-specific" and late-embryogenesis proteins. We conclude that these proteins are present in cells prior to germination and that they are subsequently used as a nutritional source for the young tissues. Native UV absorbing peptides were detected in very low molecular weight fractions and sequenced. Each peptide shared C-terminal residues and showed homology to the seed storage protein legumin. The strategies used here would be suitable for combining bioassays and mass spectrometry to identify bioactive peptides in the M. truncatula peptidome.