Tetanus Toxin <i>cis</i> -Loop Contributes to Light-Chain Translocation

How protein toxins translocate their catalytic domain across a cell membrane is the least understood step in toxin action. This study utilized a reporter, β-lactamase, that was genetically fused to full-length, nontoxic tetanus toxin (βlac-TT) in discovery-based live-cell assays to study LC translocation. Directed mutagenesis identified a role for K 768 in LC translocation. K 768 was located between α15 and α16 (termed the cis -loop). Cellular assays showed that K 768 did not interfere with other toxin functions, including cell binding, intracellular trafficking, and pore formation. The equivalent K 768 is conserved among the clostridial neurotoxin family of proteins as a conserved structural motif. The cis -loop appears to contribute to LC translocation.