A special envelope separates extra-chromosomal from chromosomal DNA in the cytoplasm of mammalian cells
Expression from transfected plasmid DNA is generally transient, but little do we know on what limits this. Live-cell imaging revealed that DNA transfected into mammalian cells was either captured directly in the cytoplasm, or was soon expelled from the nucleus, upon its entry. In the cytoplasm, plasmid DNA was rapidly surrounded by a double membrane and frequently colocalized with extra-chromosomal DNA of telomeric origin, also expelled from the nucleus. Therefore, we termed this long-term maintained structure exclusome. The exclusome envelope contains endoplasmic reticulum proteins, the inner-nuclear membrane proteins Lap2{beta} and Emerin but differs from the nuclear envelope by the absence of the Lamin B Receptor, nuclear pore complexes and by the presence of fenestrations. Further, Emerin affects the frequency of cells with exclusomes. Thus, cells wrap chromosomes and extra-chromosomal DNA into similar yet distinct envelopes. Thereby, they distinguish, sort, cluster, package, and keep extra-chromosomal DNA in the exclusome but chromosomal DNA in the nucleus, where transcription occurs.