A cyclic lipopeptide produced by an antagonistic bacterium relies on its tail and transient receptor potential-type Ca2+ channels to immobilize a green alga

The antagonistic bacterium Pseudomonas protegens secretes the cyclic lipopeptide (CLiP) orfamide A, which triggers a Ca2+ signal causing rapid deflagellation of the microalga Chlamydomonas reinhardtii.We performed chemical synthesis of orfamide A derivatives and used an aequorin reporter line to measure their Ca2+ responses. Immobilization of algae was studied using a modulator and mutants of transient receptor potential (TRP)-type channels.By investigating targeted synthetic orfamide A derivatives, we found that N-terminal amino acids of the linear part and the terminal fatty acid region are important for the specificity of the Ca2+-signal causing deflagellation. Molecular editing indicates that at least two distinct Ca2+-signaling pathways are triggered. One is involved in deflagellation (Thr(3) change, fatty acid tail shortened by 4C), whereas the other still causes an increase in cytosolic Ca2+ in the algal cells, but does not cause substantial deflagellation (Leu(1) change, fatty acid hydroxylation, fatty acid changes by 2C). Using mutants, we define four TRP-type channels that are involved in orfamide A signaling; only one (ADF1) responds additionally to low pH.These results suggest that the linear part of the CLiP plays one major role in Ca2+ signaling, and that orfamide A uses a network of algal TRP-type channels for deflagellation.

Automated summary

The antagonistic bacterium Pseudomonas protegens secretes the cyclic lipopeptide orfamide A, which triggers a Ca2+ signal causing rapid deflagellation of the microalga Chlamydomonas reinhardtii. By investigating targeted synthetic orfamide A derivatives, the researchers found that the N-terminal amino acids of the linear part and the terminal fatty acid region are important for the specificity of the Ca2+ signal causing deflagellation.