Biology Reference
In-Depth Information
A substrate is targeted for degradation by the proteasome through addition of a
polyubiquitin chain to the substrate. This reaction is catalyzed by a three-step
enzymatic cascade involving an ubiquitin-activating enzyme (E1), a ubiquitin-
targeting enzyme (E2), and a ubiquitin ligase protein (E3). The ubiquitin E3 ligase
confers substrate specificity to the reaction (Nakayama and Nakayama
2006
).
Cullin-RING ubiquitin ligases (CRLs) are members of the largest family of
eukaryotic E3-ubiquitin ligases (Petroski and Deshaies
2005
). All CRLs consist of
a cullin-backbone, a zinc-binding RING-domain containing protein which recruits
the ubiquitin-conjugating E2 enzyme, and an adaptor protein which recruits
interchangeable substrate recognition subunits. There are seven human cullin
subunits (CUL1, -2, -3, -4A, -4B, -5, and -7) responsible for nucleating the
assembly of unique E3-ubiquitin ligase complexes (Petroski and Deshaies
2005
).
In the case of the prototypical CRL, the SKP1-CUL1-F-box (SCF) ubiquitin
ligase complex, SKP1 acts as the adaptor protein interacting with substrate
recognition subunits containing an F-box domain and a second protein-protein
interaction domain which recognizes the specific target protein. Other CRLs
contain unique adaptor proteins which recognize substrate recognition subunits
with different functional motifs (Petroski and Deshaies
2005
).
Core components of the SCF E3-ubiquitin ligase complex, including SKP1 and
CUL1, have been found to localize to maternal centrioles which serve as assembly
platforms for oncogene-induced centriole overduplication (Korzeniewski et al.
2009
; Freed et al.
1999
). SCF-ubiquitin ligase activity was found to be critically
involved in suppressing centriole multiplication in human tumor cells by regulating
PLK4 protein levels (Korzeniewski et al.
2009
; Cunha-Ferreira et al.
2009
; Rogers
et al.
2009
). This activity of the SCF ubiquitin ligase provides an important mech-
anism for restraining excessive daughter centriole formation at single maternal
centrioles and hence centrosome-mediated cell division errors and chromosomal
instability (Korzeniewski et al.
2009
). Interestingly, HPV-16 E7 expression induces
centriole multiplication in a phenotype reminiscent of SCF-ubiquitin ligase inacti-
vation, suggesting that HPV-16 E7 expression may also deregulate PLK4 protein
level, half-life, or localization to induce centriole multiplication.
12.12 Mechanism of HPV-16 E7-Induced Centriole
Multiplication
HPV-16 E7 is known to deregulate cyclin E/CDK2 complexes; however, whether
the HPV-16 E7 oncoprotein deregulates PLK4 protein expression to ultimately
stimulate centriole multiplication had not been determined. When PLK4 protein
expression was analyzed in HPV-16 E7 expressing cells, it was found that PLK4
aberrantly localized to maternal centrioles in the form of multiple PLK4 dots
similar to the aberrant localization of PLK4 seen following deregulation of CDK2/
cyclin E complexes alone (Korzeniewski et al.
2011
). However, upregulation of
