摘要：Knowledge on domain swapping in vitro is increasing, but domain swapping may not occur regularly in vivo , and its information in cells is limited. Herein, we show that domain-swapped oligomers of a thermostable c -type cytochrome, Hydrogenobacter thermophilus cyt c 552, are formed in E. coli which expresses cyt c 552. The region containing the N-terminal α-helix and heme was domain-swapped between protomers in the dimer formed in E. coli . The amount of cyt c 552 oligomers increased in E. coli as the cyt c 552 concentration was increased, whereas that of high-order oligomers decreased in the order of decrease in protein stability, indicating that domain swapping decreases in cells when the protein stability decreases. Apo cyt c 552 was detected in the cyt c 552 oligomer formed in E. coli , but not in that of the A5F/M11V/Y32F/Y41E/I76V mutant. The cyt c 552 oligomer containing its apo protein may form at the periplasm, since the apo protein detected by mass measurements did not contain the signal peptide. These results show that domain-swapped cyt c 552 oligomers were formed in E. coli , owing to the stability of the transient oligomer containing the apo protein before heme attachment. This is an indication that exceedingly stable proteins may have disadvantages forming domain-swapped oligomers in cells.
