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Investigations of the intracellular distributions showed homogenous distribu-
tion of PA28
α
and PA28
β
concentrations in both cytosol and nucleus, while
PA 2 8
γ
was almost exclusively found in the nucleus (143). PA28
α
and -
β
acti-
vate all three proteolytic activities of the 20S proteasome, whereas PA28
γ
only
activates the trypsin-like activity on
β
2 (144). Therefore, the PA28
γ 7 complexes
only enhance the trypsin-like proteolytic activity on
2 about 10- to 12-fold.
The other two subunits are inhibited (from 0.4- to 0.6-fold of normal activity)
(145). This might be due to conformational changes of the
β
5-subunits.
Between subunits of the PA28 complex, a 35% similarity in their amino acid
sequences exists (146). Each of the subunits contains four long helices. A loop
between helix 2 and helix 3 is important for attachment to the
β
1- and
β
-ring of the
proteasome and proteasomal activation. The amino acids 141-149 (binding of
the regulator particle to the 20S proteasome) are highly conserved, as well as
the C-termini 240-249 (also binding to the 20S proteasome). Mutants in this
region might either inhibit the binding of PA28 to the proteasome or block
the activation of the proteasome by PA28 (147-149).
PA 2 8
α
α
and -
β
peptides are inducible by IFN-
γ
, while PA28
γ
is not. Treat-
ment with IFN-
-
subunits in HeLa, Raji, and Jurkat cells (150). The distributions of the three
PA28 isoforms in tissues are still under discussion and, in part, inconsistent,
though it seems to be clear that PA28
γ
showed a three- to fivefold increase of the PA28
α
- and
β
are mainly found in immuno-
competent tissues like thymus, spleen, and lung, while almost undetectable in
brain and nervous tissue (138).
The stoichiometry of the PA28
α
and -
β
complex was discussed for some time.
Chemical cross-linked activator complexes led to the assumption of an
α
/
β
α 3 β 3
complex first, with an alternating arrangement of the single subunits (151, 152).
But Zhang et al. found an
,
which was confirmed by mass spectrometry (36, 153). Further investigations
led to the finding of several forms of the PA28, so until now complexes PA
α 3 β 4 structure containing a
β
-
β
dimer, but no
α
-
α
α 3 β 3 ,
PA 2 8
α 4 β 3 , PA28
α 3 β 4 (in all cases with alternating arrangement of the PA28
α
-
and PA28
β
-subunits), and the homoheptamer PA28
γ 7 are known (138). In vitro
experiments mixing PA28
α
and -
β
using an
α
/
β
ratio of 1.2 revealed that the
α 4 β 3 are found with
a predominant formation of the first structure (153). Yet pure solutions of
PA 2 8
most stable form is the
α 3 β 4 arrangement, and
α 3 β 4 and
α
resulted in the formation of PA28
α 7 —very unstable and not found in
the cells structure. PA28
γ
only forms homoheptamers, while PA28
α
and -
β
bind strongly between each other and form only weak homopolymers (144).
Interestingly, in pure solutions, PA28
is not able to form any homopolymers
and is found in solution as a monomer, whereas PA28
β
is capable of associat-
ing as previously mentioned. It is also interesting to note that the affinity of
the different PA28 subunits to the 20S core proteasome is different and in the
order PA28
α
αβ
(as a heteropolymer)
>
PA28
γ
>
PA 2 8
α
>
PA 2 8
β
(145). The
complete PA28
α 3 β 4 complex is barrel shaped and has dimensions of about
60 Å in height and a diameter of about 90 Å at the base, which is binding to
an
-ring of the 20S proteasome. It has a central opening with a distal diameter
of about 20 Å and a base diameter of about 30 Å. Binding of PA28
α
α 3 β 4 induces
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