In vivo affinity label of a protein expressed in Escherichia coli

Abstract
When Tyr‐307 of the β subunit of f1 ‐ATPase from a thennophilic Bacillus strain PS3 is replaced by cysteine and expressed in Escherichia coli cells, about a half population of the mutant β subunit are labeled by Coenzyme A at Cys‐307 through a disulfide bond which is cleavable by reducing treatment. The mutant β subunit can be reconstituted into the α3β3, complex of which ATPase activity is stimulated two‐fold by reducing treatment either prior or after reconstitution. Since Tyr‐307 has been supposed to be located at one of subdomains which form the ATP binding site of the β subunit, Coenzyme A binds to the mutant β subunit as an AT(D)P analogue in E. coli cells and then covalently attaches to Cys‐307.