Insights into mechanisms and significance of domain swapping from emerging examples in the Mog1p/PsbP-like fold

Three-dimensional (3D) domain swapping in proteins occurs when identical polypeptide chains exchange structural elements to form a homo-oligomeric protein. Domain swapping can play a regulatory role for certain oligomeric proteins and has been implicated in deleterious protein aggregation. Here, we examine recently reported 3D domain swapping in proteins that contain the Mog1p/PsbP-like fold, which is a small fold found in non-enzymatic proteins that participate in a variety of distinct cellular processes. This fold was initially identified from structures of the yeast Mog1p protein, which regulates nuclear protein transport in eukaryotes, and PsbP proteins, which are part of photosystem II in plants, green algae, and cyanobacteria. The core structural element of the Mog1p/PsbP-like fold is an α-β-α sandwich that contains a 6- or 7-stranded antiparallel β-sheet. Additionally, most Mog1p/PsbP-like proteins contain an N-terminal β-hairpin that interacts with the α-β-α sandwich. Interestingly, domain-swapped dimers can form by exchange of this N-terminal β-hairpin in certain proteins. We discuss biochemical mechanisms and explore the functional significance of domain-swapping in the formation of an interaction interface in homo-dimers that bind a protein target. Lastly, we examine domain swapping between 2 tandem Mog1p/PsbP-like domains in a multidomain protein. In summary, this review provides recent examples of domain-swapping in proteins containing the Mog1p/PsbP-like fold and highlights general roles for domain-swapping in facilitating protein-protein interactions and in the evolution of multidomain proteins.