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Fig. 1. Schematic representations of different cullin RING ubiquitin ligases and
the APC/C. All cullin complexes are Nedd8 modified (see
Box 2) and recruit E2-bound
ubiquitin through their association with the RING-finger protein Rbx1.
(A) The SCF complex is the best-characterized cullin ligase. Cullin 1
proteins bind at their N terminus to the BTB-fold protein Skp1. Skp1, in
return, interacts with different F-box proteins, which specifically recognize
substrates (blue). (B) Cullin 2-based ECS complexes recruit their
substrate through their interaction with elongin B and elongin C and a
variable SOCS-box protein, such as ZIF-1 (hence the name ECS complex).
(C) Cullin 3-based complexes are unique in using only one protein to
bind the cullin and the substrate. (D) The APC/C has many more subunits
than the cullin ligases, many of which have not been assigned a function and
are shown as a single yellow mass. Although not Nedd8 modified, APC2 has
sequence similarity to cullin proteins and interacts with the APC11
RING-finger protein. Different WD40 repeat proteins, e.g. Cdc20 or Cdh1,
mediate substrate recognition by the APC/C. APC/C, anaphase promoting
complex/cyclosome; BTB, Bric-a-Brack, Tramtrack and Brahma; Cdc20, cell
division cycle 20; Cdh1, Cdc20 homolog 1; ECS, elongin B/C, cullin 2, SOCS
box; MEI-1, meiosis defective 1; MEL-26, maternal effect lethal; Rbx1, ring
box protein 1; SCF, Skp1, cullin 1 and F-box subunits; Skp1, suppressor of
kinetochore protein 1; ZIF-1, zinc-finger interacting factor 1.
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