Spatial proteomics reveals lipid droplet reorganization in symbiotic Paramecium bursaria cells

Endosymbiosis is an important adaptive mechanism allowing organisms to exploit novel niches. The flexible relationship between the ciliate Paramecium bursaria and green algae represents a model system for studying early endosymbiosis evolution. However, the mechanisms underlying how P. bursaria cells maintain this endosymbiotic relationship remain unclear. Here, we use mass spectrometry-based proteomics to generate a spatial proteomic atlas of the host P. bursaria cells with and without endosymbionts. This atlas defines the protein composition of the endosymbiont-containing compartment (perialgal vacuole) and reveals pronounced remodeling of host lipid droplets upon symbiosis. Imaging analyses confirm that lipid droplets change in size, morphology, and positioning, accumulating near intracellular algae. Perturbing symbiotic cells with chemical inhibitors of lipid metabolism reduces endosymbiotic algal numbers, revealing a role for lipid droplets in the host-endosymbiont interaction. Our data provide a comprehensive resource for protein localization in P. bursaria cells and elucidate how those cells remodel an existing cellular compartment to maintain endosymbiosis.