Structural and dynamic properties of incomplete immunoglobulin-like fold domains

The immunoglobulin fold (Ig-fold) is a widespread structural motif that is detected in a variety of proteins involved in diversified biological processes. The Ig-fold contains 70-110 residues that are assembled in a characteristic sandwich-like structure formed by two facing β-sheets each made of antiparallel β-strands. A number of variations on this common theme have been detected and described (Ig-like fold). One of the most intriguing variants is characterized by the lack of a strand compared to the canonical motif (incomplete Ig-like fold). Interestingly, proteins exhibiting incomplete Ig-like fold have been shown to play an important role in mediating either protein-protein or domain-domain interactions. Protein-protein interactions mediated by incomplete Ig-like folds play a key structural role in the chaperone-usher pathway, a process that generates multi-protein assemblies essential for the adhesion of gram negative bacteria. Domains with incomplete Ig-like fold have also been discovered in the mechanism of action of adhesins belonging to the family of MSCRAMMs (microbial surface components recognizing adhesive matrix molecules). Recently, a stable incomplete Iglike fold has been detected in the peptidoglycan-binding extra-cellular portion of Staphylococcus aureus PrkC, an important Ser/Thr membrane kinase involved in bacterial growth and revival from latency. It is important to note that the occurrence of proteins with incomplete Ig-like fold is often related to cell adhesion and infectivity of bacterial pathological agents. We here report a survey of the structural data available on this peculiar structural motif highlighting analogies and differences of incomplete Ig-like fold involved in different processes. The dynamical behavior of these domains, investigated by molecular dynamics techniques, will be also commented.