Journal Article

A contractile injection system stimulates tubeworm metamorphosis by translocating a proteinaceous effector

Eisenstein, Fabian, Shikuma, Nicholas J, Ericson, Charles F, Malter, Kyle E, Zeller, Robert W, Cavalcanti, Giselle S, Pilhofer, Martin, Medeiros, João M, Newman, Dianne K

2019

Overview

The swimming larvae of many marine animals identify a location on the sea floor to undergo metamorphosis based on the presence of specific bacteria. Although this microbe–animal interaction is critical for the life cycles of diverse marine animals, what types of biochemical cues from bacteria that induce metamorphosis has been a mystery. Metamorphosis of larvae of the tubeworm Hydroides elegans is induced by arrays of phage tail-like contractile injection systems, which are released by the bacterium Pseudoalteromonas luteoviolacea. Here we identify the novel effector protein Mif1. By cryo-electron tomography imaging and functional assays, we observe Mif1 as cargo inside the tube lumen of the contractile injection system and show that the mif1 gene is required for inducing metamorphosis. Purified Mif1 is sufficient for triggering metamorphosis when electroporated into tubeworm larvae. Our results indicate that the delivery of protein effectors by contractile injection systems may orchestrate microbe–animal interactions in diverse contexts. Many marine animals, including corals and tubeworms, begin life as larvae swimming in open water before transforming into adults that anchor themselves to the seabed. These transformations, known as metamorphoses, are often triggered by certain types of bacteria that form friendly relationships (or “symbioses”) with the animals. One such symbiosis forms between a bacterium called Pseudoalteromonas luteoviolacea and a tubeworm known as Hydroides elegans. Previous studies have shown that P. luteoviolacea produces syringe-like structures known as Metamorphosis Associated Contractile structures (or MACs for short) that are responsible for stimulating metamorphosis in the tubeworm larvae. Some viruses that infect bacteria use similar structures to inject molecules into their host cells. However, it was not clear whether MACs were also able to inject molecules into cells. Here, Ericson, Eisenstein et al. used a technique called cryo-electron tomography combined with genetic and biochemical approaches to study how the MACs of P. luteoviolacea trigger metamorphosis in tubeworms. The experiments identified a protein in the bacteria named Mif1 that was required for the tubeworms to transform. The bacteria loaded Mif1 into the tube of the MAC structure and then injected it into the tubeworms. Further experiments showed that inserting Mif1 alone into tubeworms was sufficient to activate metamorphosis. Mif1 is the first protein from bacteria to be shown to activate metamorphosis, but it is likely that many more remain to be discovered. Since other marine animals also form symbioses with bacteria, understanding how Mif1 and other similar proteins work may inform efforts to restore coral reefs and other fragile ecosystems, and increase the production of oysters and other shellfish. Furthermore, MACs and related structures may have the potential to be developed into biotechnology tools that deliver drugs and other molecules directly into animal cells.

• Publisher: eLife Science Publications, Ltd,eLife Sciences Publications Ltd,eLife Sciences Publications, Ltd
• Subject: Animal behavior / Animals / Bacteria / Bacterial Proteins - metabolism / Biochemistry / Biofilms / Biology / Biophysics / Biotechnology / Cells (Biology) / contractile injection system / Contractility / Coral reefs / Corals / Cryoelectron Microscopy / development / Ecosystems / Electron Microscope Tomography / Environmental aspects / Genes / Genetic research / Genomics / Genotype & phenotype / Host Microbial Interactions / Injection systems / Life cycles / Marine animals / Marine bacteria / Marine biology / Marine ecosystems / Marine organisms / Mass spectrometry / Metamorphosis / Metamorphosis (Biology) / Metamorphosis, Biological / Microbiology and Infectious Disease / Novels / Ocean bottom / Ocean floor / Oysters / phage / Phages / Physiological aspects / Polychaeta - drug effects / Polychaeta - growth & development / Polychaeta - microbiology / Protein Transport / Proteins / Pseudoalteromonas - metabolism / Reefs / Scientific imaging / Short Report / Structural Biology and Molecular Biophysics / Swimming / symbiosis / T6SS / Tomography / Tubeworms / Zoological research