Functional analysis of Wolbachia Cid effectors unravels cooperative interactions to target host chromatin during replication

Abstract

Wolbachia are common bacteria among terrestrial arthropods. These endosymbionts transmitted through the female germline manipulate their host reproduction through several mechanisms whose most prevalent form called Cytoplasmic Incompatibility -CI- is a conditional sterility syndrome eventually favoring the infected progeny. Upon fertilization, the sperm derived from an infected male is only compatible with an egg harboring a compatible Wolbachia strain, this sperm leading otherwise to embryonic death. The Wolbachia Cif factors CidA and CidB responsible for CI and its neutralization function as a Toxin-Antitoxin system in the mosquito host Culex pipiens. However, the mechanism of CidB toxicity and its neutralization by the CidA antitoxin remain unexplored. Using transfected insect cell lines to perform a structure-function analysis of these effectors, we show that both CidA and CidB are chromatin interactors and CidA anchors CidB to the chromatin in a cell-cycle dependent-manner. In absence of CidA, the CidB toxin localizes to its own chromatin microenvironment and acts by preventing S-phase completion, independently of its deubiquitylase -DUB- domain. Experiments with transgenic Drosophila show that CidB DUB domain is required together with CidA during spermatogenesis to stabilize the CidA-CidB complex. Our study defines CidB functional regions and paves the way to elucidate the mechanism of its toxicity.

Author summary

Wolbachia are common Rickettsiales among terrestrial arthropods species and are transmitted vertically through the female germline. These endosymbionts can cause Cytoplasmic Incompatibility -CI-, which is their major form of reproductive manipulation in insects. This sterility syndrome favors the spread of Wolbachia in the host population because eggs fertilized by a sperm from an infected male only develop properly when the female is infected with the same or a compatible Wolbachia strain. In contrast, if the sperm derived from an infected male fertilizes a non-infected egg or an egg harboring a non-compatible Wolbachia strain, CI leads to embryonic death. Culex pipiens mosquitos are infected with Wolbachia wPip endosymbionts. wPip Cid effectors causing CI function as a Toxin (CidB) /Antixotin (CidA) system. CidB is loaded without CidA in the maturing sperm to cause CI, while CidA neutralizes CidB during spermatogenesis up to the histone-to-protamine transition and in the compatible Wolbachia-infected egg. We explored through mutagenesis approaches how CidB causes cellular toxicity and how it is prevented by CidA. This allowed us to identify CidB and CidA functional domains including chromatin binding domains found in both effectors. CidA transports, stabilizes and neutralizes CidB on the chromatin, preventing the toxin to otherwise block S-phase completion.