Exosome complex

Source: Wikipedia, the free encyclopedia.
Not to be confused with Exosome (vesicle).

"Ribbon view" of the human exosome complex. PDB 2NN6 See the legend below. The channel through which RNA passes during degradation is visible at the center of the protein complex

The exosome complex (or PM/Scl complex, often just called the exosome) is a multi-

eukaryotic cells and archaea, while in bacteria a simpler complex called the degradosome
carries out similar functions.

The core of the exosome contains a six-membered ring structure to which other proteins are attached. In eukaryotic cells, the exosome complex is present in the

3′ end
in this case), and in eukaryotes also an endoribonucleolytic function, meaning it cleaves RNA at sites within the molecule.

Several proteins in the exosome are the target of

motor neuron disease
.

Discovery

The exosome was first discovered as an

RNase in 1997 in the budding yeast Saccharomyces cerevisiae, an often-used model organism.[1] Not long after, in 1999, it was realized that the exosome was in fact the yeast equivalent of an already described complex in human cells called the PM/Scl complex, which had been identified as an autoantigen in patients with certain autoimmune 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 components in the complex.[3][4] In 2001, the increasing amount of genome data that had become available allowed the prediction of exosome proteins in archaea, although it would take another 2 years before the first exosome complex from an archaeal organism was purified.[5][6]

Structure

Core proteins

Top and side view of the crystal structure of the human exosome complex. See the full legend below.

The core of the complex has a ring structure consisting of six proteins that all belong to the same class of RNases, the RNase PH-like proteins.[7] In archaea there are two different PH-like proteins (called Rrp41 and Rrp42), each present three times in an alternating order. Eukaryotic exosome complexes have six different proteins that form the ring structure.[8][9] Of these six eukaryotic proteins, three resemble the archaeal Rrp41 protein and the other three proteins are more similar to the archaeal Rrp42 protein.[10]

Subunits and organisation of the archaeal (left) and eukaryotic (right) exosome complexes. Different proteins are numbered, showing that the archaeal exosome contains 4 different proteins, but the eukaryotic exosome contains nine different proteins. See the full legend below.

Located on top of this ring are three proteins that have an S1 RNA binding domain (RBD). Two proteins in addition have a K-homology (KH) domain.[7] In eukaryotes, three different "S1" proteins are bound to the ring, whereas in archaea either one or two different "S1" proteins can be part of the exosome (although there are always three S1 subunits attached to the complex).[11]

This ring structure is very similar to that of the proteins

tRNA processing, forms a hexameric ring consisting of six identical RNase PH proteins.[12][13]
In the case of PNPase, which is a phosphorolytic RNA-degrading protein found in
3' end of RNA molecules.[7]

Associated proteins

Besides these nine core exosome proteins, two other proteins often associate with the complex in eukaryotic organisms. One of these is Rrp44, a hydrolytic RNase, which belongs to the

DIS3L1) and the other in the nucleus (DIS3).[19][20]

β-sheets in yellow.

The second common associated protein is called Rrp6 (in yeast) or PM/Scl-100 (in human). Like Rrp44, this protein is a hydrolytic exoribonuclease, but in this case of the RNase D protein family.[21] The protein PM/Scl-100 is most commonly part of exosome complexes in the nucleus of cells, but can form part of the cytoplasmic exosome complex as well.[22]

Regulatory proteins

Apart from these two tightly bound protein subunits, many proteins interact with the exosome complex in both the cytoplasm and nucleus of cells. These loosely associated proteins may regulate the activity and specificity of the exosome complex. In the cytoplasm, the exosome interacts with AU-rich element (ARE) binding proteins (e.g. KRSP and TTP), which can promote or prevent degradation of mRNAs. The nuclear exosome associates with RNA binding proteins (e.g. MPP6/Mpp6 and C1D/Rrp47 in humans/yeast) that are required for processing certain substrates.[7]

In addition to single proteins, other protein complexes interact with the exosome. One of those is the cytoplasmic Ski complex, which includes an RNA helicase (Ski2) and is involved in mRNA degradation.[23] In the nucleus, the processing of rRNA and snoRNA by the exosome is mediated by the TRAMP complex, which contains both RNA helicase (Mtr4) and polyadenylation (Trf4) activity.[24]

Function

Enzymatic function

Reaction diagrams for both hydrolytic (left) and phosphorolytic (right) 3' end degradation of RNA.

As stated above, the exosome complex contains many proteins with ribonuclease domains. The exact nature of these ribonuclease domains has changed across evolution from bacterial to archaeal to eukaryotic complexes as various activities have been gained and lost. The exosome is primarily a 3'-5'

3' end. Exoribonucleases contained in exosome complexes are either phosphorolytic (the RNase PH-like proteins) or, in eukaryotes, hydrolytic (the RNase R and RNase D domain proteins). The phosphorolytic enzymes use inorganic phosphate to cleave the phosphodiester bonds – releasing nucleotide diphosphates. The hydrolytic enzymes use water to hydrolyse these bonds – releasing nucleotide monophosphates
.

In archaea, the Rrp41 subunit of the complex is a phosphorolytic exoribonuclease. Three copies of this protein are present in the ring and are responsible for the activity of the complex.[9] In eukaryotes, none of the RNase PH subunits have retained this catalytic activity, meaning the core ring structure of the human exosome has no enzymatically active protein.[25] Despite this loss of catalytic activity, the structure of the core exosome is highly conserved from archaea to humans, suggesting that the complex performs a vital cellular function. In eukaryotes, the absence of the phosphorolytic activity is compensated by the presence of the hydrolytic enzymes, which are responsible for the ribonuclease activity of the exosome in such organisms.[26][27][28]

As stated above, the hydrolytic proteins Rrp6 and Rrp44 are associated with the exosome in yeast and in humans, besides Rrp6, two different proteins, Dis3 and Dis3L1 can be associated at the position of the yeast Rrp44 protein.[19][20] Although originally the S1 domain proteins were thought to have 3'-5' hydrolytic exoribonuclease activity as well, the existence of this activity has recently been questioned and these proteins might have just a role in binding substrates prior to their degradation by the complex.[26]

Schematic view of the archaeal (left) and eukaryotic (right) exosome complexes with the most common associated proteins. In color and marked with a star are the subunits of each complex that have catalytic activity. See below for a full legend.

Substrates

The exosome is involved in the degradation and

small nucleolar RNAs.[1][32][33]

Although most cells have other enzymes that can degrade RNA, either from the

5' end of the RNA, the exosome complex is essential for cell survival. When the expression of exosome proteins is artificially reduced or stopped, for example by RNA interference, growth stops and the cells eventually die. Both the core proteins of the exosome complex, as well as the two main associated proteins, are essential proteins.[34] Bacteria do not have an exosome complex; however, similar functions are performed by a simpler complex that includes the protein PNPase, called the degradosome.[35]

Two core subunits of the archaeal exosome (Rrp41 and Rrp42), bound to a small RNA molecule (in red).

The exosome is a key complex in cellular RNA quality control. Unlike prokaryotes, eukaryotes possess highly active RNA surveillance systems that recognise unprocessed and mis-processed RNA-protein complexes (such as

protein synthesis.[36]

In addition to RNA processing, turnover and surveillance activities, the exosome is important for the degradation of so-called cryptic unstable transcripts (CUTs) that are produced from thousands of loci within the yeast genome.[37][38] The importance of these unstable RNAs and their degradation are still unclear, but similar RNA species have also been detected in human cells.[39]

Disease

Autoimmunity

The exosome complex is the target of

enzyme linked immunosorbent assays (ELISAs) for detecting these antibodies.[7]

In these diseases, antibodies are mainly directed against two of the proteins of the complex, called PM/Scl-100 (the RNase D like protein) and PM/Scl-75 (one of the RNase PH like proteins from the ring) and antibodies recognizing these proteins are found in approximately 30% of patients with the PM/Scl overlap syndrome.[43] Although these two proteins are the main target of the autoantibodies, other exosome subunits and associated proteins (like C1D) can be targeted in these patients.[44][45] At the current time, the most sensitive way to detect these antibodies is by using a peptide, derived from the PM/Scl-100 protein, as the antigen in an ELISA, instead of complete proteins. By this method, autoantibodies are found in up to 55% of patients with the PM/Scl overlap syndrome, but they can also be detected in patients with either scleroderma, polymyositis, or dermatomyositis alone.[46]

As the autobodies are found mainly in patients that have characteristics of several different autoimmune diseases, the

Raynaud's phenomenon, arthritis, myositis and scleroderma.[47] Treatment of these patients is symptomatic and is similar to treatment for the individual autoimmune disease, often involving either immunosuppressive or immunomodulating drugs.[48]

Cancer treatment

The exosome has been shown to be inhibited by the

tumors. In yeast cells treated with fluorouracil, defects were found in the processing of ribosomal RNA identical to those seen when the activity of the exosome was blocked by molecular biological strategies. Lack of correct ribosomal RNA processing is lethal to cells, explaining the antimetabolic effect of the drug.[49]

Neurological disorders

Mutations in

motor neuron disease, cerebellar atrophy, progressive microcephaly and profound global developmental delay, consistent with pontocerebellar hypoplasia type 1B (PCH1B; MIM 614678).[50]

List of subunits

Legend General name Domains Human Yeast (S. cerevisiae) Archaea
MW
(kD) 		Human gene Yeast gene
1 Csl4 S1 RBD hCsl4 Csl4p/Ski4p Csl4 21–32 EXOSC1 YNL232W
2 Rrp4 S1/KH RBD hRrp4 Rrp4p Rrp4 28–39 EXOSC2 YHR069C
3 Rrp40 S1/KH RBD hRrp40 Rrp40p (Rrp4)A 27–32 EXOSC3 YOL142W
4 Rrp41 RNase PH hRrp41 Rrp41p/Ski6p Rrp41C 26–28 EXOSC4 YGR195W
5 Rrp46 RNase PH hRrp46 Rrp46p (Rrp41)A,C 25–28 EXOSC5 YGR095C
6 Mtr3 RNase PH hMtr3 Mtr3p (Rrp41)A,C 24–37 EXOSC6 YGR158C
7 Rrp42 RNase PH hRrp42 Rrp42p Rrp42 29–32 EXOSC7 YDL111C
8 Rrp43 RNase PH OIP2 Rrp43p (Rrp42)A 30–44 EXOSC8 YCR035C
9 Rrp45 RNase PH PM/Scl-75 Rrp45p (Rrp42)A 34–49 EXOSC9 YDR280W
10 Rrp6 RNase D PM/Scl-100C Rrp6pC n/a 84–100 EXOSC10 YOR001W
11 Rrp44 RNase R Dis3B,C

Dis3L1B,C

Rrp44p/Dis3pC n/a 105–113 DIS3

DIS3L1

 YOL021C

See also

References

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Further reading

External links

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