BCH3042 Lecture 16: Bcl-2 Identification
25 May 2018
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Lecture 16 – Bcl-2 Identification
Bcl-2
• Bcl-2 (B cell lymphoma-2) gene discovered at the t(14;18) chromosome
translocation point in 90% of B-cell follicular lymphomas
o Translocation of 14 is switched with chromosome 18
o B-Cells secrete antibodies
• Bcl-2 transcription upregulated by its
repositioning downstream of the IgG heavy
chain promoter and enhancer
o But found Bcl-2 doesn’t promote cell
proliferation like most oncogenes but
rather its overexpression prevents
apoptosis
• Tumour promotion can involve activation of
oncogenes coupled with
dysregulation/inactivation of apoptosis (anti-
apoptotic)
o Bcl-2 isn’t secreted
o Wouldn’t be expressed in liver
• Bcl-2 found at mitochondrial outer membrane (MOM)
o Does overexpression of Bcl2 block cyt c release following cell death
signal?
o PARP isn’t cleaved in presence of Bcl-2
o Cyt c is retained in mitochondria in presence of Bcl-2 and in cytosol in
absence of Bcl-2
Bcl-2 Family Guardians and Effectors
• Guardians: multi-domain pro-survival proteins
o N→C terminus
o E.g. Bcl-2, Bcl-X
o Prevent effectors from acting through
direct interactions
▪ Binding is mediated by BH3
domain
o Form heterodimers with Bax or Bak and send it back to cytosol
(inactive)
o Survival > death apoptosis
• Effectors: Bax and Bak – make holes in MOM
o Function interchangeably: both can make holes
o Knock out Bak-/- mice will have a stronger phenotype in Bax
Bax and Bak Locations
• Bax is cytosolic, targeted to MOM after activation
o Undergoes conformational changes
o Moves from cytosol → mitochondria during apoptosis
• Bak is integrated in the MOM, normally inactive (bound to protein receptor,
VDAC2) until apoptotic activation
• Active Bak and Bax form oliogmers that lead to cyt c release
Document Summary
Bcl-2 family guardians and effectors: guardians: multi-domain pro-survival proteins, n c terminus, e. g. Bax and bak locations: bax is cytosolic, targeted to mom after activation, undergoes conformational changes, moves from cytosol mitochondria during apoptosis, bak is integrated in the mom, normally inactive (bound to protein receptor, Vdac2) until apoptotic activation: active bak and bax form oliogmers that lead to cyt c release. Bh3-only members of bcl-2 family: = apoptotic initiators tip the balance towards apoptosis, e. g. How bh3-only proteins bind guardians: salt bridge between proteins and prevents job, hydrophobic region, all bcl-2 family effector/guardians have overall similar shape, globular helix bundle with hydrophobic surface groove, bh3 only proteins bind to groove, e. g. Bax/bak to become activator: can directly activate bax/bak through transient interactions, both lead to cyt c release and cell death. Direct activation of bak and bax by bh3 only proteins: bh3 only proteins (e. g. tbid) can also transiently interact and directly activate effectors as well.
