S. Elias et al., DEGRADATION OF ORNITHINE DECARBOXYLASE BY THE MAMMALIAN AND YEAST 26SPROTEASOME COMPLEXES REQUIRES ALL THE COMPONENTS OF THE PROTEASE, European journal of biochemistry, 229(1), 1995, pp. 276-283
Ornithine decarboxylase (ODC), a key enzyme in the biosynthesis of pol
yamines, is an extremely short-lived protein. This attribute is import
ant for the regulation of the activity of the enzyme and implies that
the mechanisms involved in its degradation play an important role in t
he control of the cellular processes in which the enzyme is involved.
Recently, it has been shown that ODC is degraded by the 26S proteasome
complex in a process that requires antizyme, but not ubiquitin. With
one reported exception, ODC, the 26S complex recognizes and degrades s
pecifically ubiquitinated proteins. Their unconjugated counterparts ar
e not targeted. The 26S complex is composed of a core catalytic unit,
the 20S proteasome complex, and two additional, and apparently distinc
t, subcomplexes. The two additional subcomplexes are regulatory subuni
ts that are required in order to confer specificity and control. In th
is study, we demonstrate that, like the degradation of ubiquitin-conju
gated proteins, ubiquitin-independent degradation of ODC also requires
prior assembly of the mammalian 26S proteasome from all the three sub
units, the 20S proteasome and the two subcomplexes. The combination of
any two subunits does not support generation of a proteolytically act
ive complex. This is also true for the yeast 26S complex. Like the mam
malian 20S proteasome, the yeast 20S complex can cleave short peptides
in an ATP-independent mode, but cannot degrade ODC or ubiquitin-conju
gated proteins. These proteins are degraded only following addition of
the regulatory subunits and generation of the high-molecular-mass 26S
complex. In a distinct, but related, set of experiments, we demonstra
te that the degradation of ODC by the assembled 26S proteasome in vitr
o reproduces faithfully proteolysis of the enzyme in the intact cell.
Namely, (a) a C-terminal-deleted mouse ODC and trypanosome ODC are sta
ble both in vitro and in vivo, and (b) like proteolysis in the intact
cell, degradation in the reconstituted cell-free system is also depend
ent upon the addition of antizyme.