Cytoplasmic adaptor proteins (CAPS) in mast cell signaling

Author: Margarita Martín
Biochemistry Unit. Faculty of Medicine
University of Barcelona. 08036 Barcelona, Spain

What are adaptor proteins?

Proteins with no enzymatic activity that mediate intermolecular interactions through motifs* and /or domains** and are essential in organizing, coordinating and compartmentalizing activated signaling molecules.
*Motif: short sequence of aminoacids
**Domain: conserved part of a given protein sequence and structure that can evolve, function, and exist independently of the rest of the protein chain.

Types of adaptor proteins

TRAPS: Transmembrane adaptor proteins
CAPS: Cytoplasmic adaptor proteins, possessing no hydrophobic transmembrane domain. They are localized free in the cytosol or are associated with the cytoskeleton, membranes and organelles.

Important domains in CAPS

Src-homology 2 SH2: Binds specific phosphotyrosine (pY)-containing motifs in the context of
3 or 6 aminoacids located carboxy terminal to the pY.
Src-homology 3 SH3: Binds proline-rich sequences in a left-handed polyproline type II helix.
Phosphotyrosine binding PTB: Binds pY-containing peptide motifs/Asp-Pro-any aminoacid-pY).
Pleckstrin homology PH: Binds specific phosphoinositides and thus mediate translocation to plasma membrane.
Proline rich PxxP: Aminoacid sequences rich in proline residues able to bind SH3 domains.
ENA/VASP homology domain EVH1: 160-190 aminoacids in lenght. Responsible for  F-actin binding.

Expression and function of CAPS in mast cells (Figure 1 and 2)

Cytosolic adaptor proteins are essential for signal propagation from the plasma membrane receptors.
In the case of the IgE high affinity receptor (FcεRI), CAPS such as 3BP2, SLP76, GADS, function to
assemble macromolecular complexes and promote, coordinate and control FceRI–mediated
mast cell responses including degranulation and de novo synthesis of cytokines and eicosanoids (1).


3BP2  (SH3 Binding Protein 2)

3BP2 (85 kDa)
Abundantly expressed in hematopoietic cells.
Ligands in mast cells: LAT1 (2),  Syk  (3), PLCγ (3), Lyn (4), Vav, 14-3-3 (5).
Function: Overexpression of SH2 domain of 3BP2 in the rat mast cell line RBL-2H3 inhibits Ca2+ influx and degranulation (2).
Silencing of 3BP2 in human mast cells blocks calcium influx, degranulation and cytokine production (3).


SLP76 (SH2 domain containing leukocyte-specific phosphoprotein 76kDa)

SLP-76 (76 kDa)
Expression: Abundantly expressed in hematopoietic cells.
Ligands in mast cells:  GADS (6), Vav, SLAP-130 (7), Grb2 (8).
Function: SLP-76 –null mast cells are defective in degranulation and cytokine secretion (9). The in vivo response is also defective (9).

GADs  (Grb2-related adaptor downstream of Shc)

GADS ( 40kDa)
Expression: Abundantly expressed in hematopoietic cells.
Ligands in mast cells: SLP76 (6), LAT 1 (7).
Function:  Similar to SLP-76 since recruitment  of GADS (via SH2 domain) to the LAT 1 protein complex after FcεRI stimulation is essential
for SLP-76 function (6).


Grb2 (Growth factor receptor–bound protein 2)

Grb2 (25 kDa)
Expression: Ubiquitous.
Ligands in mast cells: SOS, Shc (10), LAT 1 (11), LAT 2 (12), Gab2 (13), SLP-76 (8).
Function: The pathway Grb2-SOS induces MAP kinase activation after FceRI stimulation but no direct evidence of function in mast cells (10).



Shc (46, 52 kDa)
Expression: Ubiquitous. Two isoforms expressed on mast cells.
Ligands in mast cells: SHIP1 (14), Grb2 (15), Gab2 (13).
Function: Amplifies the signaling of Grb2-SOS activating MAP kinases. Also plays an important role in the negative regulation of FceRI
binding to phosphatase SHIP-1. The phosphatase may compete with Grb2 binding  to Shc interfering with SOS recruitment and therefore MAP activation (16).


SLAP-130 (SLP-76 associated protein 130 kDa) (alternative names: ADAP/ Fyb/FynB)
SLAP-130 (130 kDa)
Expression: T and mast cells.
Ligands in mast cells: SLP76 (7), Fyn (19),  SKAP55 (18).
Function: In the RBL-2H3 mast cell line overexpression of SLAP-130 increases adhesion to fibronectin and enhances mast cell degranulation (18,19). However ,SLAP-130 deficiency does no alter degranulation nor affect adhesion to fibronectin in vitro. Anaphylaxis is unaltered in SLAP-130 null-mice (21).


Gab2 (Grb2-binding protein 2)
Gab2 (97 kDa)                                                             
Expression: Ubiquitous.
Ligands in mast cells: p85 (22), Grb2, Shc (13), PLC-g1, PLC-g2, SHP-2 ( 23), Fyn (24).
Function: Gab2-null mast cells shows impaired degranulation upon FceRI stimulation and are defective in cytokine secretion. Accordingly, Gab2-null mice shows disminished responses in cutaneous and systemic anaphylaxis. Gab2 deficiency causes a defect in PI3K activation (22).  Unexpectedly, Gab2 overexpression causes suppression of FceRI signaling possibly through binding to SHP-2 (22,23).


DOK (Downstream of tyrosine kinase)

DOK (62kDa)
Expression: T and mast cells. Different isoforms Dok1, Dok2 and Dok3 in mast cells.
Ligands in mast cells: RasGAP (25), SHIP-1 (26).
Function: Dok1 forms a negative regulatory complex  with SHIP-1/RasGAP downregulating PI3K and MAPKinase pathways. The complex was described in mast cells after FceRI and FcgRIIB co-aggregation (26) as well as CD84 triggering (27). Dok1deficiency does not affect mast cell activation, suggesting a possible redundant role of different isoforms expressed (26).


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