CASP9 (Caspase-9) Notes - ISOFORMS Project

Key Isoform Biology

CASP9 is an initiator caspase in the intrinsic apoptosis pathway, but has a dominant-negative short isoform.

Isoforms (4 named)

Isoform UniProt ID Synonym Key Feature Function
Isoform 1 P55211-1 9L, Alpha Full-length, catalytically active INDUCES APOPTOSIS
Isoform 2 P55211-2 9S, Beta Lacks catalytic domain DOMINANT-NEGATIVE INHIBITOR
Isoform 3 P55211-3 Gamma - Unknown
Isoform 4 P55211-4 - - Unknown

Critical Antagonism: Long vs Short

Caspase-9L (Isoform 1):
- Contains CARD domain for Apaf-1 binding
- Contains catalytic domain
- Forms apoptosome with Apaf-1 and cytochrome c
- Cleaves and activates CASP3/CASP7
- PRO-APOPTOTIC

Caspase-9S (Isoform 2):
UniProt explicitly states:

"Lacks activity is an dominant-negative inhibitor of caspase-9" PMID:10070954

Mechanism: Caspase-9S competes with Caspase-9L for Apaf-1 binding but cannot activate downstream caspases.

Mechanism of Antagonism

Apaf-1 + Cyt c + dATP → Apoptosome

If Caspase-9L binds: → Apoptosis
If Caspase-9S binds: → No apoptosis (dominant-negative)

The ratio of 9L/9S determines apoptotic sensitivity.

GOA Annotation Status

Expected Annotation Issues

  1. "Caspase activity" - TRUE for isoform 1, FALSE for isoform 2
  2. "Positive regulation of apoptotic process" - TRUE for isoform 1, FALSE for isoform 2
  3. "Negative regulation of apoptotic process" - should be annotated for isoform 2!
  4. "Apoptosome" annotations - both isoforms can be in apoptosome, but with opposite effects

Biological Significance

Like BCL2L1, this is a case where the balance between isoforms determines cell fate:
- High 9L/9S ratio → apoptosis-sensitive
- Low 9L/9S ratio → apoptosis-resistant

Cancer cells often shift this ratio towards 9S to evade apoptosis.

Key References