R bodies

R bodies (from refractile bodies, also R-bodies) are polymeric protein inclusions formed inside the cytoplasm of bacteria.^[1] Initially discovered in kappa particles, bacterial endosymbionts of the ciliate Paramecium, R bodies (and genes encoding them) have since been discovered in a variety of taxa.^[2]

At neutral pH, type 51 R bodies resemble a coil of ribbon approximately 500 nm in diameter and approximately 400 nm deep.^[1] Encoded by a single operon containing four open reading frames,^[3][4] R bodies are formed from two small structural proteins, RebA and RebB.^[5] A third protein, RebC, is required for the covalent assembly of these two structural proteins into higher-molecular weight products, visualized as a ladder on an SDS-PAGE gel.^[5]

At low pH, Type 51 R bodies undergo a dramatic structural rearrangement. Much like a paper yo-yo, the ribbon extends (from the center) to form hollow tube with pointed ends that can reach up to 20μm in length.^[6]

Other types of R bodies from different bacterial species vary in their size, ribbon morphology, and triggers for extension.^[1]

When kappa particles shed from a killer paramecium are ingested, R bodies extend within the acidic food vacuole of the predatory paramecium, distending and rupturing the membrane.^[7] This liberates the contents of the food vacuole into the cytoplasm of the paramecium.^[7] While feeding kappa particles to sensitive paramecium results in the death of paramecium, feeding purified R bodies or R bodies recombinantly expressed in E. coli is not toxic.^[3][8] Thus, R bodies are thought to function as a toxin delivery system.

R bodies are also capable of rupturing E. coli spheroplasts, demonstrating that they can rupture membranes in a foreign context, and they can be engineered to extend at a variety of different pH levels.^[9]