PiQSi is a community effort that facilitates the investigation and curation of quaternary structures. Given a PDB identifier or a protein sequence, it displays information about the quaternary structure of homologous proteins on a single web-page. This information allows a quick comparison of the quaternary structure(s) of the members of a protein family. So far, the web-server has allowed the manual curation of ~15000 structures of PDB Biological Units, of which about 15% were found to be likely errors (e.g. guilty).
Illustration of PiQSi information
Each protein can be displayed along with its homologs of known structure, and information that helps to determine the biological oligomeric state.
Levy ED. PiQSi: protein quaternary structure investigation. Structure. 2007 Nov;15(11):1364-7. doi: 10.1016/j.str.2007.09.019. PMID: 17997962.