Proteolysis restricts localization of CID, the centromere-specific histone H3 variant of Drosophila, to centromeres, Nucleic acids research

Centromere identity is determined by the formation of a specialized chromatin structure containing the centromere-specific histone H3 variant CENP-A. The precise molecular mechanism(s) accounting for the specific deposition of CENP-A at centro-meres are still poorly understood. Centromeric deposition of CENP-A, which is independent of DNA replication, might involve specific chromatin assembly complexes and/or specific interactions with kinetochore components. However, transiently expressed CENP-A incorporates throughout chro-matin indicating that CENP-A nucleosomes can also be promiscuously deposited during DNA replica-tion. Therefore, additional mechanisms must exist to prevent deposition of CENP-A nucleosomes dur-ing replication and/or to remove them afterwards. Here, using transient expression experiments per-formed in Drosophila Kc cells, we show that proteasome-mediated degradation restricts localiza-tion of Drosophila CENP-A (CID) to centromeres by eliminating mislocalized CID as well as by regulating available CID levels. Regulating available CID levels appears essential to ensure centromeric deposition of transiently expressed CID as, when expression is increased in the presence of proteasome inhibitors, newly synthesized CID mislocalizes. Mislocaliza-tion of CID affects cell cycle progression as a high percentage of cells showing mislocalized CID are reactive against aPSer10H3 antibodies, enter mitosis at a very low frequency and show strong segregation defects. However, cells showing reduced amounts of mislocalized CID show normal cell cycle progression