MICAN-SQ: a sequential protein structure alignment program that is applicable to monomers and all types of oligomers.

Motivation: Protein structure alignment is a significant tool to understand evolutionary processes and physicochemical properties of proteins. Important targets of structure alignment are not only monomeric but also oligomeric proteins that sometimes include domain swapping or fusions. Although various protein structural alignment programs have been developed, no method is applicable to any protein pair regardless of the number of chain components and oligomeric states with retaining sequential restrictions: structurally equivalent regions must be aligned in the same order along protein sequences. Results: In this paper, we introduced a new sequential protein structural alignment algorithm MICAN-SQ, which is applicable to protein structures in all oligomeric states. In particular, MICAN-SQ allows the complicated structural alignments of proteins with domain swapping or fusion regions. To validate MICAN-SQ, alignment accuracies were evaluated using curated alignments of monomers and examples of domain swapping, and compared with those of pre-existing protein structural alignment programs. The results of this study show that MICAN-SQ has superior accuracy and robustness in comparison with previous programs and offers limited computational times. We also demonstrate that MICAN-SQ correctly aligns very large complexes and fused proteins. The present computations warrant the consideration of MICAN-SQ for studies of evolutionary and physicochemical properties of monomeric structures and all oligomer types. Availability and implementation: The MICAN program was implemented in C. The source code and executable file can be freely downloaded from http://www.tbp.cse.nagoya-u.ac.jp/MICAN/.