# Protrichocysts: a hybrid defense extrusive organelle bridging mechanical projection and chemical secretion in ciliates

## TL;DR

This study reveals that protrichocysts in ciliates are complex organelles combining mechanical and chemical defense, with roles in signaling and regeneration.

## Contribution

The first detailed characterization of protrichocyst structure, function, and regeneration, linking ciliate biology to broader eukaryotic secretory systems.

## Key findings

- Protrichocysts combine mechanical projection and chemical secretion in a three-stage ejection process.
- Regeneration of protrichocysts occurs within 12 hours via ribosomal and carboxypeptidase gene activity.
- Comparative multi-omics show conserved features between ciliate protrichocysts and metazoan dense-core granules.

## Abstract

•Protrichocysts launches a millisecond-scale, three-stage ejection that unites trichocyst-like mechanical ejection with toxicyst-like chemical release, providing ciliates an inducible hybrid defense against predators.•First comprehensive histochemical and proteomic map of its protrichocysts uncovers acid mucopolysaccharides, microtubules and candidate toxic proteins, implying roles beyond defense such as intercellular signaling.•Protrichocysts fully regenerate within 12 h after experimental ablation, driven by a transcriptional burst of ribosomal and carboxypeptidase genes that mirror metazoan dense-core-granule biogenesis.•Comparative multi-omics reveal conserved molecular architectures between these ciliate extrusomes and metazoan exosomes/dense-core granules, highlighting a unified eukaryotic secretory heritage.

Protrichocysts launches a millisecond-scale, three-stage ejection that unites trichocyst-like mechanical ejection with toxicyst-like chemical release, providing ciliates an inducible hybrid defense against predators.

First comprehensive histochemical and proteomic map of its protrichocysts uncovers acid mucopolysaccharides, microtubules and candidate toxic proteins, implying roles beyond defense such as intercellular signaling.

Protrichocysts fully regenerate within 12 h after experimental ablation, driven by a transcriptional burst of ribosomal and carboxypeptidase genes that mirror metazoan dense-core-granule biogenesis.

Comparative multi-omics reveal conserved molecular architectures between these ciliate extrusomes and metazoan exosomes/dense-core granules, highlighting a unified eukaryotic secretory heritage.

Extrusive organelles (extrusomes) represent a specialized class of dense-core granules in protists that exhibit remarkably complex morphology and secretory mechanisms compared to their metazoan counterparts, reflecting key adaptations in single-celled eukaryote evolution. Protrichocysts, which are among the most intricate projectile extrusomes in ciliates, have thus far been understood primarily in terms of their general morphology. In this study, we employed a comprehensive approach to investigate the protrichocysts of Pseudourostyla cristata, thereby expanding our understanding of these organelles. Through predator-prey interaction experiments and electron microscopy, we demonstrated the ejection of protrichocysts in response to a predator’s attack and revealed that their ejection process consists of three main stages: (i) cap dissolution, (ii) dual-phase body expansion, and (iii) controlled shaft exposure. Histochemical analysis indicated that extruded protrichocysts contain acid mucopolysaccharides and microtubules. SDS-PAGE and HPLC-MS/MS analyses identified the major protein bands and the presence of glycoproteins, phosphoproteins, and potential defense-related proteins. Single-cell transcriptome analysis suggested the involvement of conserved carboxypeptidase-mediated extrusome biogenesis. These findings establish protrichocysts as multifunctional organelles that employ a unique hybrid defense strategy combining mechanical projection with chemical secretion. Their distinctive microtubular cap architecture and specialized matrix composition further suggest potential roles in intercellular communication. Additionally, this research improves our understanding of protist extrusome biology and provides new insights into the evolution of eukaryotic secretory systems.

Image, graphical abstract