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)

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

• 109 total annotations
• NO isoform-specific annotations (no P55211-X identifiers)
• Annotations conflate pro-apoptotic 9L with anti-apoptotic 9S

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

• PMID:10070954 - Caspase-9S dominant-negative function

2026-09-30 APOPTOSIS review

• Re-reviewed the pre-existing CASP9 YAML without using the provider deep-research
  files as assertion support. Primary rows now cite cached PubMed, Reactome, GOA,
  and GO-CAM evidence directly.
• Tightened inherited top-level GO:0006915 apoptotic process rows to
  GO:0097193 intrinsic apoptotic signaling pathway, and tightened broad
  apoptosis-regulation assertions to GO:1900119 positive regulation of execution phase of apoptosis where the evidence is full-length CASP9 maturing
  executioner procaspases.
• Retained GO:0051604 protein maturation for the direct CASP9 step of
  proteolytically activating CASP3/CASP7; CASP9 is upstream of the execution
  phase rather than itself being an executioner-caspase component.
• Replaced APaf-1/procaspase-9 generic GO:0005515 rows with
  GO:0050700 CARD domain binding or the apoptotic cysteine-endopeptidase
  activator term as appropriate, and removed uninformative high-throughput or
  inhibitor/interactor GO:0005515 edges.
• Made the endogenous Caspase-9S negative-regulator proposal isoform-specific
  to P55211-2 and more precise as GO:2001243 negative regulation of intrinsic apoptotic signaling pathway; removed a broad GO:0030234 enzyme regulator activity placeholder that had only been present to satisfy the core-function
  shape.
• Deferred abstract-only rows where the visible record did not expose the
  CASP9 assay behind the GOA annotation, and marked the DNA-damage/c-Abl Tyr-153
  rows as unresolved because PMID:29066624 challenged the original activating
  phosphorylation model from PMID:15657060.