Biology Reference
In-Depth Information
but rather use a RING domain containing protein to bring an E2 into intimate
association with substrates [reviewed in (Rotin and Kumar
2009
; Skaar and
Pagano
2009
). RING-type ligases are multi subunit complexes that consist of a
core scaffold that bridges a protein containing an E2-binding RING domain with a
variety of targeting factors that bind to substrates. In the Skp/Cullin/F-box (SCF)
E3 ligase, Cul1 acts as a scaffold for Skp1, which recruits both the RING domain
protein Rbx1 and one of several F-box proteins that determines substrate speci-
ficity [reviewed in (Skaar and Pagano
2009
)]. Because F-box proteins typically
bind to phosphorylated proteins, substrate phosphorylation is generally the trigger
for SCF-dependent degradation. In the anaphase promoting complex/cyclosome
(APC/C) E3 ligase, the Cullin-like Apc2 protein serves as a scaffold for eleven
other subunits, including the RING domain protein Apc11 that recruits the E2, and
either Cdc20 or Cdh1 that determine substrate specificity (van Leuken et al.
2008
).
Phosphorylation can also regulate the binding of substrates to the APC/C, although
unlike the SCF, phosphorylation more often prevents APC/C-dependent degra-
dation (Simpson-Lavy et al.
2010
).
8.4 Centrosomes as a Site of Action for the Proteasome
In 1993 the proteasome was seen to associate with spindle fibers (Amsterdam et al.
1993
), leading to the suggestion that it regulated the cell cycle by degrading
mitotic cyclins, and was later observed to associate with mitotic spindle poles
(Wojcik et al.
1995
). In 1996 it was recognized that treatment of cells with
proteasome inhibitors promotes the formation of large cytoplasmic aggregates that
contained both proteasome components and ubiquitylated proteins. These aggre-
gates were located at a perinuclear region in close proximity with Golgi and were
hypothesized to represent cellular proteolysis centers (Wojcik et al.
1996
).
Subsequently termed aggresomes (Johnston et al.
1998
), these proteolysis centers
were shown to be highly regulated structures containing proteasomes and chap-
erones that are assembled at the centrosome in response to misfolded or damaged
proteins (Corboy et al.
2005
) through dynein-mediated microtubule-dependent
transport (Johnston et al.
2002
). The association of proteasomes with MTOCs is
evolutionarily conserved, as it was recently shown that aggresomes form and
colocalize with the spindle pole body (SPB) when exon 1 of the human Huntington
protein is expressed in budding yeast (Wang et al.
2009
). Many cellular proteins
destined for degradation have been found at centrosomes, including p53 and
Hsp70 in cells expressing adenoviral E1A/E1B proteins (Brown et al.
1994
), and
misfolded nucleoprotein from the influenza virus (Anton et al.
1999
). However,
this is not unique to cell cycle regulators targeted for degradation in virally
infected cells, and in 1998 it was found that misfolded cystic fibrosis transmem-
brane conductance regulator accumulated at centrosomes, together with the
molecular chaperones thought to be responsible for its presentation to the pro-
teasome (Johnston et al.
1998
; Loo et al.
1998
).
