The activity and stability of a cold-active acylaminoacyl peptidase rely on its dimerization by domain swapping.

The study of enzymes from extremophiles arouses interest in Protein Science because of the amazing solutions these proteins adopt to cope with extreme conditions. Recently solved, the structure of the psychrophilic acyl aminoacyl peptidase from Sporosarcina psychrophila (SpAAP) pinpoints a mechanism of dimerization unusual for this class of enzymes. The quaternary structure of SpAAP relies on a domain-swapping mechanism involving the N-terminal A1 helix. The A1 helix is conserved among homologous mesophilic and psychrophilic proteins and its deletion causes the formation of a monomeric enzyme, which is inactive and prone to aggregate. Here, we investigate the dimerization mechanism of SpAAP through the analysis of chimeric heterodimers where a protomer lacking the A1 helix combines with a protomer carrying the inactivated catalytic site. Our results indicate that the two active sites are independent, and that a single A1 helix is sufficient to partially recover the quaternary structure and the activity of chimeric heterodimers. Since catalytically competent protomers are unstable and inactive unless they dimerize, SpAAP reveals as an "obligomer" for both structural and functional reasons.