Photoaffinity Labeling of Creatine Kinase with 2-Azido- and 8-Azidoadenosine Triphosphate: Identification of Two Peptides from the ATP-Binding Domain

Abstract

Two different analogs of ATP, [gamma-32P]2N3ATP and and [gamma-32P]8N3ATP, were used to photoaffinity label the MM and BB isoforms of rabbit cytosolic creatine kinase. Evidence that photoinsertion was within the ATP-binding domain was as follows: (1) Assays for creatine phosphate production demonstrated that [gamma-32]2N3ATP and [gamma-32P]8N3ATP are substrates for creatine kinase. (2) Enzymatic activity was inhibited by photolabeling with either analog. (3) Saturation of photoinsertion was observed for both analogs. Half-maximal saturation was observed at 5 microM [gamma-32P]2N3ATP or 12 microM (gamma-32P]8N3ATP. (4) Photoinsertion of both probes could be decreased by micromolar levels of ATP. Immobilized Al3+ affinity chromatography and HPLC were used to isolate the peptides modified by these probes. Overlapping sequence analysis of the isolated peptides from the tryptic and chymotryptic digests of the photolabeled MM isoform revealed that [gamma-32P]8N3ATP photoinserted into the peptide region corresponding to Val279-Arg291, whereas [gamma-32P]2N3-ATP photoinserted into Val236-Lys241. The corresponding peptide (Ile279-Arg291 and Val236-Lys241) from the BB isoform were shown to be selectively modified. We conclude that amino acid residues within the peptide regions 236-241 and 279-291 of rabbit cytosolic creatine kinase are localized within the binding domain for the adenine moiety of ATP. The results also demonstrate the effectiveness and selectivity of Al3+ as the chelating agent in immobilized metal affinity chromatography for the isolation of photolabeled peptides as well as its potential to enhance retention of radiolabel during HPLC.