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Pactolus I‐domain: Functional switching of the Rossmann fold

Murine Pactolus is a neutrophil‐specific single chain glycoprotein that plays a role as an apoptosis marker for macrophages. The extracellular region of the protein shows strong sequence similarities to integrin β‐subunits. Critical sequence modifications differentiate its function when compared to the integrin family. We show experimentally that Pactolus I‐domain does not bind divalent metal ions, indicating that ligand binding is not mediated through a metal ion‐dependent adhesion site (MIDAS). NMR data was used to map secondary structure and the strand pairing within the β‐sheet to confirm an overall Rossmann fold topology. Homology modeling enhanced by the NMR data was used to determine the overall structure, with two key loop insertions/deletions (insertion 2 and SDL) that distinguish the Pactolus I‐domain from the integrin α I‐domain and β I‐domains. NMR peak exchange broadening is observed due to dimerization, correlating to the β I‐domain and β propeller heterodimerization region within the integrin headpiece. Two unique N‐linked glycosylation sites (Asn151 and Asn230) within this region disrupt dimerization and may account for why Pactolus is not found to associate with an α‐subunit. These changes in quaternary structure, ligand binding loops, glycosylation, and metal sites illustrate how evolution has rapidly and effectively altered key aspects of the integrin β‐subunit to derive a protein of novel function on an existing protein scaffold. Proteins 2007. © 2007 Wiley‐Liss, Inc.