FAS (CD95/APO-1) Notes - ISOFORMS Project

Key Isoform Biology

FAS is a classic example of antagonistic membrane vs soluble isoforms.

Isoforms (7 named)

Isoform	UniProt ID	Synonym	Key Feature	Function
Isoform 1	P25445-1	-	Full-length, membrane-bound	INDUCES APOPTOSIS
Isoform 2	P25445-2	del2, D	Lacks exons	Soluble, blocks apoptosis
Isoform 3	P25445-3	del3, E	Lacks exons	Soluble, blocks apoptosis
Isoform 4	P25445-4	B	-	Soluble, blocks apoptosis
Isoform 5	P25445-5	C	-	Soluble, blocks apoptosis
Isoform 6	P25445-6	TMdel, A	Lacks TM domain	Soluble, BLOCKS APOPTOSIS
Isoform 7	P25445-7	FasExo8Del	-	Unknown

Critical Antagonism: Membrane vs Soluble

Membrane-bound FAS (Isoform 1):
- Contains transmembrane domain
- Forms DISC (Death-Inducing Signaling Complex)
- Recruits FADD and CASP8
- TRIGGERS APOPTOSIS

Soluble FAS (Isoforms 2-6):
UniProt states:

"The secreted isoforms 2 to 6 block apoptosis (in vitro)"

Mechanism: Soluble FAS acts as a decoy receptor, binding FasL without triggering apoptosis.

Tissue-Specific Expression (UniProt)

"Isoform 1 and isoform 6 are expressed at equal levels in resting peripheral blood mononuclear cells. After activation there is an increase in isoform 1 and decrease in the levels of isoform 6."

This suggests splicing regulation shifts towards the pro-apoptotic membrane form upon immune activation.

GOA Annotation Status

96 total annotations
NO isoform-specific annotations (no P25445-X identifiers)
All apoptosis annotations likely refer to membrane-bound isoform 1

Expected Annotation Issues

"Positive regulation of apoptotic process" - TRUE for isoform 1, FALSE for isoforms 2-6
"Negative regulation of apoptotic process" - should be annotated for soluble isoforms!
Death domain - present in all, but only functional in membrane form

Key References

PMID:7533181, PMID:9184224 - Soluble FAS blocks apoptosis
PMID:7575433 - Isoform expression patterns