文摘
Protein splicing is a robust multistep posttranslational process catalyzed by inteins. In the Mtu RecA intein, a conserved block-F aspartate (D422) coordinates different steps in protein splicing, but the precise mechanism is unclear. Solution NMR shows that D422 has a strikingly high pKa of 6.1, two units above the normal pKa of aspartate. The elevated pKa of D422 is coupled to the depressed pKa of another active-site residue, the block-A cysteine (C1). A C1A mutation lowers the D422 pKa to normal, while a D422G mutation increases the C1 pKa from 7.5 to 8.5. The pKa coupling and NMR structure determination demonstrate that protonated D422 serves as a hydrogen bond donor to stabilize the C1 thiolate and promote the N鈥揝 acyl shift, the first step of protein splicing. Additionally, in vivo splicing assays with mutations of D422 to Glu, Cys, and Ser show that the deprotonated aspartate is essential for splicing, most likely by deprotonating and activating the downstream nucleophile in transesterification, the second step of protein splicing. We propose that the sequential protonation and deprotonation of the D422 side chain is the coordination mechanism for the first two steps of protein splicing.