The Mechanism of Molecular Recognition by the COMMD Proteins

Abstract

Regulation of protein activity is vital to prevent aberrant cell behaviour. Cells control the activity of proteins through many mechanisms, including the control of protein expression and degradation. The COMMD protein family have a subtle but nevertheless important role in the regulation of cellular protein levels, exhibiting both negative and positive regulatory effects. Through this activity, the COMMD proteins are involved in many cellular pathways, ranging from copper and sodium transport, to inflammation and the response to hypoxia. The COMMD proteins are named for a conserved C-terminal COMM domain that is present in all family members. The COMMD protein family are widely distributed throughout Eukarya, including 10 human homologues that are ubiquitously expressed in tissues. Originally identified in Bedlington terriers for their role in cooper toxicosis, they have since been found to interact with a diverse variety of proteins. The COMMD proteins appear to function by acting as an adaptor protein in large protein complexes bringing their targets together. The conserved C-terminal COMM domain mediates the interactions of the COMMD proteins. As such, this ~85 amino acid domain must recognise a range of proteins. The mechanism by which the COMMD proteins recognise their interaction partners is unknown. CCDC22 is a recently identified interaction partner of the COMMD proteins. The domain organisation of CCDC22 is unknown but the N-terminal 323 amino acids are known to associate with the COMMD proteins in vivo. Whether or not the association is direct is unknown. In this study domain organisation of CCDC22 is assessed and the in vitro interaction between COMMD1 and an N-terminal fragment of CCDC22 is investigated. There is a notable lack of structural information pertaining to the COMMD proteins. The conserved COMM domain has an unknown structure, and part of this work aims to move toward solving its structure. There are high-resolution structures for the N-terminal variable domains of both COMMD1 and COMMD9. An alternative crystal form of semet COMMD9 where the N-terminal domain adopts a domain-swapped trimer conformation has also been observed. A higher-resolution 2.4 Å native N-COMMD9 domain-swapped trimer diffraction dataset is refined and analysed in this study.