page 407, Campellone et al. reduce this complex phenomenon to twelve amino acids, showing that clustering a small domain of the bacterial Tir protein, which is translocated to the host cell, is sufficient to induce actin rearrangement. The results highlight an interesting evolutionary convergence, and provide a simple model system for studying actin assembly.
After discovering that Tir is the only E. coli component required for pedestal formation, the authors further whittled the system down to the C-terminal cytoplasmic domain of Tir. Clustering this domain at the plasma membrane causes its phosphorylation, allowing it to bind to the host protein Nck. Nck binding leads to the recruitment of N-WASP and the Arp2/3 actin nucleating complex, followed by actin pedestal formation. A 12–amino acid piece of the Tir COOH terminus triggers actin assembly in Xenopus egg extracts. Interestingly, a similar peptide sequence in vaccinia virus allows the virus to be transported on actin tails, suggesting that two unrelated pathogens have evolved to exploit Nck in similar ways. The authors now hope to determine how Nck binding to the Tir sequence leads to actin rearrangement. ▪