Crystal Structure of PotD, the Primary Receptor of the Polyamine Transport System in Escherichia coli*

PotD protein is a periplasmic binding protein and the primary receptor of the polyamine transport system, which regulates the polyamine content in Escherichia coli. The crystal structure of PotD in complex with sper-midine has been solved at 2.5-Å resolution. The PotD protein consists of two domains with an alternating b-a-b topology. The polyamine binding site is in a cen-tral cleft lying in the interface between the domains. In the cleft, four acidic residues recognize the three posi-tively charged nitrogen atoms of spermidine, while five aromatic side chains anchor the methylene backbone by van der Waals interactions. The overall fold of PotD is similar to that of other periplasmic binding proteins, and in particular to the maltodextrin-binding protein from E. coli, despite the fact that sequence identity is as low as 20%. The comparison of the PotD structure with the two maltodextrin-binding protein structures, deter-mined in the presence and absence of the substrate, suggests that spermidine binding rearranges the rela-tive orientation of the PotD domains to create a more compact structure. Polyamines, such as putrescine, spermidine, and spermine, are ubiquitous in all living organisms. They are involved in a wide variety of biological reactions, including nucleic acid and protein synthesis (1, 2). These compounds exist as linear molecules with two (putrescine; NH