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TABLE 2.1
Subunits of the 20S Proteasome a
20S “Core” Proteasome
Systematic
Saccharomyces
cerevisiae
Homo sapiens
Mass (kDa)
α 1
C7/Prs2
HsPROS27/HsIota
27.5
α 2
Y7
HsC3
25.9
Y13
HsC9
29.5
α 3
α 4
Pre6
HsC6/XAPC7
27.9
α 5
Pup2
HsZeta
26.4
α 6
Pre5
HsC2/HsPROS30
30.2
α 7
C1/Prs1
HsC8
28.4
β 1
Pre3
HsDelta/Y
25.3 (21.9)
β 1 i
Lmp2
23.2 (20.9)
Pup1
Z
20.0 (24.5)
β 2
β 2 i
Mecl1
28.9 (23.8)
β 3
Pup3
HsC10-II
22.9
β 4
C11/Pre1
HsC7-I
22.8
β 5
Pre2
X/MB1
N/A (22.4)
β 5 i
Lmp7
30.4 (21.2)
β 6
C5/Prs3
HsC5
26.5 (23.3)
Pre4
HsN3/HsBPROS26
29.2 (24.4)
β 7
a
According to Coux et al., Annu. Rev. Biochem. 65: 801-847, 1996 and Jung et al., Mol. Aspects
Med. 30: 191-296, 2009.
The molecular masses of the proteasome are according to Coux et al. (Annu. Rev. Biochem. 65:
801-847, 1996). The numbers in brackets indicate the individual molecular masses before post-
transcriptional processing of the subunit. The systematic names of the single subunits were defined
according to their position (α- or β-ring) in the mature proteasome. An “i” in the systematic names
shows the γ-interferon-“inducible” proteasomal subunits (β1, β2, and β5).
limited unfolding of the proteasome, as performed for many years with the
addition of low doses of sodium dodecylsulfate (SDS), lead to an increase of
proteolytic activity of the treated proteasomes due to an SDS-induced opening
of the gate to the proteolytic chamber (55). Other approaches such as repeated
freeze-thaw cycles or conditions of low ionic strength lead to the same effect.
These results were pointing to an involvement of structural changes of the
gating proteasome subunits (36). It is possible that other low-molecular-weight
substances also physiologically induce such an unfolding-related activation.
However, physiologically, most relevant is the opening of the gate by binding
of one of the regulatory subunits (like 11S or 19S) to the proteasomal
-rings,
resulting in an increased activity (up to 10-fold) (56, 57) and a changed
substrate specificity. Furthermore, and especially important for the degrada-
tion of oxidized proteins, the binding of hydrophobic amino acid patches (as
occurring in misfolded or oxidatively unfolded/damaged proteins) is sufficient
to induce the conformational changes in the gating structures of the
α
α
-rings
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