The Retinoblastoma Protein Its Structure and Its Role in Cancer

The Retinoblastoma Protein
Its Structure and Its Role in Cancer
By Ariel Lefkovith
Breast Cancer
Cervical Cancer
Colo-rectal cancer
Endometrial cancer
Head & neck cancer
Liver cancer
Lung cancer
Malignant carcinoid
Malignant glioma
Urothelial cancer
Malignant lymphoma
Malignant melanoma
Ovarian cancer
Pancreatic cancer
Prostate cancer
Renal cancer
Skin cancer
Stomach cancer
Testis cancer
Thyroid cancer
It is a protein product of the RB1 gene.
A nuclear phosphoprotein.
Gene cloned in 1986.
Small cell lung carcinoma and Retinoblastoma
are cancers in which the RB1 gene is mutated
and RB1 loss of heterozygosity, forming Rb-/Rb-,
is found in other cancers.
• A tumor suppressor because of its control over
the cell cycle.
• Some viral oncoproteins can bind to pRb to
deactivate it.
pRB Role in the Cell Cycle
• One of the factors needed to pass the restriction point
from G1 phase to S phase.
• pRB is bound to the E2F transcription factor.
• pRB is phosphorylated by different cyclin-Cdk complexes
at different stages of the cell cycle.
• In G0 quiescent cells pRB is unphosphorylated and
• In early G1 pRB is bound to E2F and becomes
hypophosporylated by cyclin D-Cdk 4/6.
• In late G1 pRB is bound to E2F and becomes
hyperphosphorylated by cyclin E-Cdk 2.
• The hyperphosphorylation of the pRB inactivates it,
releasing E2F.
The Cycle
• Three main Domains: N-terminal Domain,
C-terminal Domain, and Pocket Domain.
• The N-terminal domain has 379 residues.
• The Pocket is made of 406 residues.
• The C-terminal domain is a 143 residue
• Has 16 phosphorylation sites.
The Pocket
• Made up by A and B domains.
• The two main structures are the A and B cyclin-box folds
of five α helices each.
• There are also 8 other α helices, a beta hairpin, and an
extended tail.
• The A-B Interface is held together by a hydrophobic
core, created by 20 side chains and hydrogen bonds
between the back bone and side chains.
• E2F binds in the pocket.
• Many of the mutations are found in the pocket.
• The LxCxE site is on the B box.
The helices in Domain A are in red.
The helices in Domain B are in green.
The N-terminal Domain
• Structure is hard to crystallize.
• Has a globular structure
formed by an A lobe and a B
• Similar structure to the Pocket
• The two lobes have separate
hydrophobic cores.
• It is hypothesized to be
associated with familial
retinoblastoma and could be
involved with protein-protein
Lobe A
Lobe B
The C-terminal Domain
• Structure hard to crystalize.
• This domain is necessary if pRB is stop cell
progression by binding to E2F.
• This domain binds to the E2F-DP heterodimer.
• Phosphorylation also deactivates this domain
from binding to the E2F-DP complex.
• Part of this domain can bind to the LxCxE site on
the pocket of the protein.
C-terminal Domain Fragment bound to
• The E2F-DP complex forms an
intermolecular coiled coil, an
intermolecular β sandwich, and
5 α helices.
• The β sandwich has a
hydrophobic core.
• The fragment of RbC forms a
strand-loop-helix structure and
a tail.
• The strand-loop of RbC binds
to the β sandwich, and the tail
loops around one end.
• RbC binds to the E2F-DP
complex through hydrogen
bonds and van der Waals
Red-RbC fragment
pRB bound to E2F
• Binds in the pRB pocket
• E2F binds residues on α4, α5,
α6, α8, and α9 from A and α11
from B.
• The ends of E2F make the
most contact with pRB.
• E2F binds to pRB by its main
chain, which hydrogen bonds
to pRB’s side chain.
• Tyr(411), Leu(424), Phe(425),
Glu(419), and Asp(423) are the
five most important E2F
•Domain A is red.
•Domain B is neon green.
E2F Residues
•Tyr(411) is orange.
•Asp(423) is blue.
•Tyr(411)-E2F has a phenolic ring in a
hydrophobic pocket of pRB, and its hydroxyl
group makes a hydrogen bond to Glu(554)pRB.
•Leu(424)-E2F and Phe(425)-E2F interact
hydrophobicly with pRB at Lys(530)-pRB and
•Glu(419)-E2F, which forms a hydrogen bond
with a water molecule and Thr(645)-pRB.
•Asp(423)-E2F, which forms a salt bridge with
•Asp(423)-E2F and Glu(419)-E2F point
outward, unlike the other three residues that
point inward.
•Glu(419) is light blue.
•Leu(424) is magenta.
•Phe(425) is green
LxCxE site
• Is a shallow groove found
on the B box of the
pocket domain.
• The groove is formed by
α17, α14, α15 on the
sides, the hydrophobic
core of the B box, and
α16 and α18 on the ends.
• HPV E7, T antigen, and
E1A oncoproteins all
have the LxCxE motif and
bind to the site to
deactivate pRB.
HPV E7 bound at LxCxE
• Alternating Leu 22, Cys 24, Glu 26
(Green), and Leu 28 side chains point into
the groove.
• The sidechains that point in the box
interact by van der Waals forces and
hydrogen bonds.
• Asp 21, Tyr 23, Tyr 25, Gln 27, and Asn 29
don’t bind because they point out of the
box (Magenta).
• Leu 22, Leu 28, and Cys 24 bind in
hydrophobic pockets of the groove