Study On The Molecular Mechanisms Of The Gamete Killer Cw9 In Fission Yeast

Gamete killers are one type of selfish genes.This type of selfish genes can skew Mendel's 1:1 transmission ratio through killing the gametes that do not carry a killer gene.Gamete killers cause hybrid sterility in animals,plants,and fungi and are regarded as an important evolutionary force for speciation.Although gamete killers have been found in many species,relatively little is known about their molecular mechanisms.In a published study,we found two gamete killers,cw9 and cw27,in CBS5557,a natural isolate of Schizosaccharomyces pombe.Both cw9 and cw27 belong to the wtf gene family,and there are 25 wtf genes in the lab strain.Members of the wtf gene family encode multi-transmembrane proteins and are up-regulated during meiosis.Active wtf genes can produce two types of transcripts.The short transcript encodes a protein toxic to all gametes.This protein is referred to as the "poison".The long transcript encodes a protein that can protect the gene carriers from the toxicity of the poison.This protein is referred to as the "antidote".We aim to understand the mechanisms of wtf gamete killers,especially the molecular mechanisms underlying the activities of the poison and the antidote.We mainly focus on the study of cw9.The poison protein,Cw9ki(the killing isoform of Cw9),has killing activity.The antidote protein,Cw9pi(the protecting isoform of Cw9),has protecting activity.By applying methods of genetics,cell biology,and molecular biology,we uncover the mechanistic basis of the protecting activity.Through the N-terminal PY motifs,Cw9pi recruits ubiquitin ligases Publ and Pub3,which ubiquitinate Cw9pi.Cw9ki can interact with Cw9pi and form a complex.This ubiquitinated complex is recognized by ubiquitin-binding trafficking adaptors Gga21 and Gga22,which promote the transport of the complex from the TGN(trans Golgi network)to the endosome.At the endosome,the ESCRT(endosomal complexes required for transport)machinery mediates the internalization of the complex into the MVB(multivesicular body).The complex is eventually transported into vacuoles for degradation.Regarding the killing mechanism,we propose a model based on a number of preliminary data.When Cw9pi is absent,Cw9ki is transported to the plasma membrane from the TGN.On the plasma membrane,Cw9ki may alter permeability,causing cellular toxicity.An important finding of our study is that the only fundamental difference between the poison and the antidote of a wtf gamete killer is the absence or presence of ubiquitin modification.By artificially controlling ubiquitin modification,a poison can be converted to an antidote and an antidote can be converted to a poison.We propose that the conversion between the poison and the antidote through post-translational modification may be a universal mechanism of gamete killers.Our study provides new insights into the mechanisms of gamete killers.