LMP7 into nascent proteasomes (40). Thus, the kinetics of protea-
some biogene sis seems to be differentially determined by the two
5 subunits and their molecular interplay with POMP, allowing a
rapid switch from c20S to i20S function.
Importantly, the rapid degradation of POMP and the strongly
accelerated maturation of i20S do not, as might have been expected,
require any additional IFN-
-induced factors. Consequently, the
dynamics of i20S formation appears to be e ssentially self-controlled
and the result of a functional interdependence of LMP7 and POMP.
Moreover, the up-regulation of i20S is a transient response because,
independent of IFN-
signaling, i20S exhibit a much shorter
half-life than c20S. Thus, both i20S-specific characteristics may be
due to intrinsic properties of the enzyme complex. As shown by
immunological experiments (27), the incorporation of LMP7 can
affect the structural properties of the proteasome. This also be-
come s apparent by the altered chromatographic properties of
The key position of POMP and LMP7 in the accelerated
formation of i20S is supported by their direct interaction. In
contrast with our experiments, an interaction of POMP with the
subunit but not with LMP7 was recently reported based on a yeast
two-hybrid interaction screen (37). This negative result was inter-
preted to reflect the observed biased incorporation of
5 and LMP7
into the re spective proteasome complexes due to their different
prosequences (40). As shown here, interaction of POMP with both
5 homologues is independent of the prosequences,
suggesting a second interaction site within domains of the matured
subunits. This is in agreement with our previous observation that
5i prosequence is not absolutely essential for its incorporation
into 20S proteasomes, and that POMP incorporation into precursor
complexe s is independent of the
5i prosequence (17). Although
5i prosequence is not essential for
5i subunit incorporation,
the presence of the correct prosequence strongly supports the
subunit’s incorporation efficiency (17).
In the presence of sufficient amounts of POMP, the availability
of LMP7 seems to be the rate-limiting factor of this process and vice
versa. Our POMP knockdown experiments demonstrate that
POMP determines the recruitment of the LMP7 subunit (and that
5) into the proteasome complex. In contrast to the role of the
yeast Ump1p homologue (20), POMP function turns out to be
essential for proteasome biogenesis and consequently also for
mammalian cell viability. As an immunological consequence, the
limited generation of antigenic peptide s also is reflected by the
reduction of MHC class I surface expression in POMP-depleted
cells (25). These data therefore demonstrate that POMP possesse s
an essential coordinative function in proteasome assembly that is
independent of the different prosequences and not directly corre-
lated with the differential incorporation of the two
-induced amounts of POMP therefore will permit an efficient
recruitment of the LMP7 subunit, which concomitantly facilitates
the accelerated formation of i20S.
The effectiveness of the MHC class I immune response of an
attacked organism is largely determined by the rapid and coordi-
nated generation of antigenic peptides and their presentation on the
cell surface. In conclusion, the accelerated formation of i20S forced
by the interplay between POMP and LMP7 meets the demands of
an efficient and rapid answer to an immunological challenge.
Subsequently, the observed strongly reduced half-life of i20S per-
mits cells to return more rapidly to a normal situation once i20S
functions is no longer needed, also supporting the finding of a
transient nature of CD8
T cell priming (26). Thus, our model is in
good agreement with the current knowledge of MHC class I antigen
presentation, following the hypothe sis that most antigenic peptide s
derived from defective ribosomal products, allowing cells to cope
immediately with rapidly replicating viruses (42). Therefore, we
present an immune-adaptation mechanism by the proteasome
system, which is essentially self-controlled and rapid enough to
contribute to an efficient immune response.
We are grateful to J. Monaco (University of Cincinnati, College of
Medicine, Cincinnati) for kindly providing the C8 antiserum, to C. Beier
for excellent technical assist ance, and to the members of the Kloetzel
laboratory for support and helpful discussions and for critically reading
the manuscript. This study was supported by the Deutsche Forschungs-
gemeinschaft (Kl 427兾8-5兾SFB 421).
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