Exosome complex
"Ribbon view" of the human exosome com-
plex. See the legend below.
The exosome complex (or PM/Scl com-
plex, often just called the exosome) is a mul-
ti-protein complex capable of degrading vari-
ous types of RNA (ribonucleic acid) mo-
lecules. Exosome complexes are found in
both eukaryotic cells and archaea, while in
bacteria a simpler complex called the degra-
dosome carries out similar functions.
The core of the exosome contains a six-
membered ring structure to which other pro-
teins are attached. In eukaryotic cells, the
exosome complex is present in the cytoplasm,
nucleus and especially the nucleolus, al-
though different proteins interact with the
exosome complex in these compartments reg-
ulating the RNA degradation activity of the
complex to substrates specific to these cell
compartments. Substrates of the exosome in-
clude messenger RNA, ribosomal RNA, and
many species of small RNAs. The exosome
has an exoribonucleolytic function, meaning
it degrades RNA starting at one end (the so-
called 3′ end in this case), rather than cleav-
ing the RNA at specific sites within the
molecule.
Although no causative relation between
the exosome complex and any disease is
known, several proteins in the exosome are
the target of autoantibodies in patients with
specific autoimmune diseases (especially the
PM/Scl overlap syndrome) and some anti-
metabolitic chemotherapies for cancer func-
tion by blocking the activity of the exosome.
Discovery
The exosome was first discovered as an
RNase in 1997 in the budding yeast Sacchar-
omyces cerevisiae, an often-used model or-
ganism.[1] Not long after, in 1999, it was
realized that the exosome was in fact the
yeast equivalent of an already described com-
plex in human cells, the so-called PM/Scl
complex, which had been identified as an
autoantigen in patients with certain autoim-
mune diseases years earlier (see below).[2]
Purification of this "PM/Scl complex" allowed
the identification of more human exosome
proteins and eventually the characterization
of all comp