Amino acids are the predominant form of nitrogen available to the heterotrophic tissues of plants.
These essential organic nutrients are transported across the plasma membrane of plant cells by proton-amino acid symporters. We are investigating the structure and function of this porter because it is a member of a large gene family in plants and because its wide expression pattern suggests it plays a central role in resource allocation. We then used in vitro translation, partial digestion with proteinase K, and immunoprecipitation to identify a family of oriented-peptide fragments. We modeled the topology of NAT2 based on the lengths of the peptide fragments that allowed us to predict the location of protease accessible cleavage sites. This model of protein topology anchors our complementary investigations of porter structure and function using site-directed and random mutagenesis.