Disease-causing SAP mutants are defective in ligand binding and protein folding.
The X-linked lymphoproliferative (XLP) syndrome is caused by mutations or deletions in the SH2D1A gene that encodes an SH2 domain protein named SH2D1A or SAP. The identification of a number of missense mutations within the protein's SH2 domain, each of which can directly cause disease, provides a unique opportunity to investigate the function of an interaction protein module, SH2, in the pathogenesis of XLP. We show here that SAP mutants found in XLP patients are defective in binding its physiological ligands signaling lymphocyte activating molecule (SLAM), a co-receptor in T cell activation, and Fyn, a Src family protein tyrosine kinase. Consequently, these mutants are deficient in signaling through the SLAM receptor. This is reflected by compromised abilities for the mutants to recruit Fyn to SLAM and to activate Fyn, by reduced phosphorylation of the receptor, and by deficiencies for the mutants in blocking binding of SHP-2 to SLAM. Furthermore, all mutants examined are defective in protein folding as manifested by their significantly reduced melting temperatures upon thermal denaturation, compared to that of SAP. Taken together, these results suggest that defects in ligand binding, receptor signaling, and protein folding collectively contribute to the loss of function for disease-causing SAP mutants.
Li
Chengjun
C
Department of Biochemistry, Faculty of Medicine and Dentistry, University of Western Ontario, London, Ontario N6A 5C1, Canada.
Iosef
Cristiana
C
Jia
Christina Y H
CY
Gkourasas
Theofanis
T
Han
Victor K M
VK
Shun-Cheng Li
Shawn
S
D000595:Amino Acid Sequence; D015703:Antigens, CD; D001483:Base Sequence; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D002460:Cell Line; D041321:Chromosomes, Human, X / Q000235:genetics; D005454:Fluorescence Polarization; D008040:Genetic Linkage; D006023:Glycoproteins / Q000378:metabolism / Q000502:physiology; D006801:Humans; D007136:Immunoglobulins / Q000378:metabolism / Q000502:physiology; D047908:Intracellular Signaling Peptides and Proteins; D008024:Ligands; D008232:Lymphoproliferative Disorders / Q000235:genetics* / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D020125:Mutation, Missense; D017510:Protein Folding; D017434:Protein Structure, Tertiary / Q000235:genetics; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051076:Proto-Oncogene Proteins c-fyn; D011956:Receptors, Cell Surface; D015398:Signal Transduction / Q000235:genetics; D000071179:Signaling Lymphocytic Activation Molecule Associated Protein; D000071177:Signaling Lymphocytic Activation Molecule Family Member 1; D018909:src Homology Domains / Q000235:genetics*
D015703:Antigens, CD; D002352:Carrier Proteins; D006023:Glycoproteins; D007136:Immunoglobulins; D047908:Intracellular Signaling Peptides and Proteins; D008024:Ligands; D011518:Proto-Oncogene Proteins; D011956:Receptors, Cell Surface; C114953:SH2D1A protein, human; D000071179:Signaling Lymphocytic Activation Molecule Associated Protein; D000071177:Signaling Lymphocytic Activation Molecule Family Member 1; D011505:Protein-Tyrosine Kinases; C494957:FYN protein, human; D051076:Proto-Oncogene Proteins c-fyn