Cancer Immune Therapy Edited by G. Stuhler and P. Walden ...

3.2
Immuno-Proteasomes
One characteristic feature of the MHC class I antigen presentation pathway is that
several of its components are induced by the cytokine interferon (IFN)-g. This includes
the MHC class I heavy chain, the transporter associated with antigen processing
(TAP) proteins, several of the 20S proteasome subunits and the proteasome activator
PA28 ±a protein complex that can influence proteasome behavior by substituting
for one of the two 19S regulator complexes [9±11].
Three of the 20S proteasome's b subunits, all of which exhibit proteolytic activity, are
IFN-g inducible: b1i (LMP2), b5i (LMP7) and b2i (MECL-1). These are referred to
as the immuno-subunits and their incorporation into the 20S core requires its de
novo assembly. In consequence, new 20S complexes are formed in which the constitutive
subunits, b1 (delta), b2 (z) and b5 (MB1), are replaced by the three immunosubunits
[12, 13].
Incorporation of the immuno-subunits seems to be a cooperative event implying
that the immuno-subunits are incorporated together guaranteeing that a defined population
of proteasome complexes with altered cleavage properties are formed [14].
The situation is, however, probably more complex than this since IFN-g not only induces
the formation of pure immuno-proteasomes, but also that of 20S complexes
in which immuno-subunits and constitutive subunits coexist [15].
3.2.1
The Function of Immuno-Proteasomes
3.2 Immuno-Proteasomes
Two different approaches were employed to further elucidate the role of the IFN-ginducible
subunits in the production of antigenic peptides. Proteasomes containing
the constitutive or facultative active-site subunits were purified from cells and tested
for reactivity against the three different categories of fluorogenic substrates that
cover the hydrolyzing specificity of the different active-site subunits [16±21].
Although these studies showed a clear reduction of the caspase-like activity (cleavage
after acidic residues) upon incorporation of immuno-subunits, only inconsistent
changes in the activities displayed by the two other active site were observed. More
importantly, a further analysis of cleavage products generated from larger polypeptides
and protein substrates demonstrated that the activities of the three different
catalytic centers towards longer substrates are less well defined [22]. Thus cleavages
after basic residues in fluorogenic tripeptides are solely mediated by the b2/b2 i pair
of catalytic subunits [23]. Cleavage after the same residues within polypeptides substrates
appears to be performed by different active-site subunits. Overall, these studies
failed to reveal the essence of immuno-subunits.
The analysis of mutant cell lines with defective LMP2 and LMP7 expression did not
reveal any major consequences of the absence of these subunits for antigen presentation
[24±26]. The restoration of TAP expression in the T2 and 712.174 lymphoblastoid
cell lines which lack the genomic region encoding TAP, LMP2 and LMP7 re-established
MHC class I surface expression and antigenic peptide presentation. Thus
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