Toll-like receptor (TLR) signaling relies on Toll/interleukin-1 receptor homology (TIR) domain-containing adaptor proteins that recruit downstream signaling molecules to generate tailored immune responses. In addition, the palmitoylated transmembrane adaptor protein family member Scimp acts as a non-TIR-containing adaptor protein in macrophages, scaffolding the Src family kinase Lyn to enable TLR phosphorylation and proinflammatory signaling responses. Here we report the existence of a smaller, naturally occurring translational variant of Scimp (Scimp TV1), which is generated through leaky scanning and translation at a downstream methionine. Scimp TV1 also scaffolds Lyn, but in contrast to full-length Scimp, it is basally rather than lipopolysaccharide (LPS)-inducibly phosphorylated. Macrophages from mice that selectively express Scimp TV1, but not full-length Scimp, have impaired sustained LPS-inducible cytokine responses. Furthermore, in granulocyte macrophage colony-stimulating factor-derived myeloid cells that express high levels of Scimp, selective overexpression of Scimp TV1 enhances CpG DNA-inducible cytokine production. Unlike full-length Scimp that localizes to the cell surface and filopodia, Scimp TV1 accumulates in intracellular compartments, particularly the Golgi. Moreover, this variant of Scimp is not inducibly phosphorylated in response to CpG DNA, suggesting that it may act via an indirect mechanism to enhance TLR9 responses. Our findings thus reveal the use of alternative translation start sites as a previously unrecognized mechanism for diversifying TLR responses in the innate immune system.

• MeSH
• DNA metabolism   MeSH
• Macrophages metabolism   MeSH
• Mice MeSH
• Signal Transduction * MeSH
• src-Family Kinases metabolism   MeSH
• Toll-Like Receptors * metabolism   MeSH
• Animals MeSH
• Check Tag
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH

Transmembrane adaptor proteins are molecules specialized in recruiting cytoplasmic proteins to the proximity of the cell membrane as part of the signal transduction process. A member of this family, SLP65/SLP76, Csk-interacting membrane protein (SCIMP), recruits a complex of SLP65/SLP76 and Grb2 adaptor proteins, known to be involved in the activation of PLCγ1/2, Ras, and other pathways. SCIMP expression is restricted to antigen-presenting cells. In a previous cell line-based study, it was shown that, in B cells, SCIMP contributes to the reverse signaling in the immunological synapse, downstream of MHCII glycoproteins. There it mainly facilitates the activation of ERK MAP kinases. However, its importance for MHCII glycoprotein-dependent ERK signaling in primary B cells has not been analyzed. Moreover, its role in macrophages and dendritic cells has remained largely unknown. Here we present the results of our analysis of SCIMP-deficient mice. In these mice, we did not observe any defects in B cell signaling and B cell-dependent responses. On the other hand, we found that, in dendritic cells and macrophages, SCIMP expression is up-regulated after exposure to GM-CSF or the Dectin-1 agonist zymosan. Moreover, we found that SCIMP is strongly phosphorylated after Dectin-1 stimulation and that it participates in signal transduction downstream of this important pattern recognition receptor. Our analysis of SCIMP-deficient dendritic cells revealed that SCIMP specifically contributes to sustaining long-term MAP kinase signaling and cytokine production downstream of Dectin-1 because of an increased expression and sustained phosphorylation lasting at least 24 h after signal initiation.

• MeSH
• Adaptor Proteins, Signal Transducing genetics   metabolism   MeSH
• B-Lymphocytes metabolism   MeSH
• Cell Line MeSH
• Dendritic Cells metabolism   MeSH
• Phospholipase C gamma genetics   metabolism   MeSH
• Lectins, C-Type genetics   metabolism   MeSH
• MAP Kinase Signaling System physiology   MeSH
• Mice, Mutant Strains MeSH
• Mice MeSH
• Animals MeSH
• Check Tag
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH

Transmembrane adaptor proteins (TRAPs) are structurally related proteins that have no enzymatic function, but enable inducible recruitment of effector molecules to the plasma membrane, usually in a phosphorylation dependent manner. Numerous surface receptors employ TRAPs for either propagation or negative regulation of the signal transduction. Several TRAPs (LAT, NTAL, PAG, LIME, PRR7, SCIMP, LST1/A, and putatively GAPT) are known to be palmitoylated that could facilitate their localization in lipid rafts or tetraspanin enriched microdomains. This review summarizes expression patterns, binding partners, signaling pathways, and biological functions of particular palmitoylated TRAPs with an emphasis on the three most recently discovered members, PRR7, SCIMP, and LST1/A. Moreover, we discuss in silico methodology used for discovery of new family members, nature of their binding partners, and microdomain localization.

• MeSH
• Adaptor Proteins, Signal Transducing metabolism   MeSH
• Leukocytes metabolism   physiology   MeSH
• Humans MeSH
• Lipoylation MeSH
• Membrane Microdomains metabolism   MeSH
• Membrane Proteins metabolism   MeSH
• Signal Transduction * MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Review MeSH

Formation of the immunological synapse between an antigen-presenting cell (APC) and a T cell leads to signal generation in both cells involved. In T cells, the lipid raft-associated transmembrane adaptor protein LAT plays a central role. Its phosphorylation is a crucial step in signal propagation, including the calcium response and mitogen-activated protein kinase activation, and largely depends on its association with the SLP76 adaptor protein. Here we report the discovery of a new palmitoylated transmembrane adaptor protein, termed SCIMP. SCIMP is expressed in B cells and other professional APCs and is localized in the immunological synapse due to its association with tetraspanin-enriched microdomains. In B cells, it is constitutively associated with Lyn kinase and becomes tyrosine phosphorylated after major histocompatibility complex type II (MHC-II) stimulation. When phosphorylated, SCIMP binds to the SLP65 adaptor protein and also to the inhibitory kinase Csk. While the association with SLP65 initiates the downstream signaling cascades, Csk binding functions as a negative regulatory loop. The results suggest that SCIMP is involved in signal transduction after MHC-II stimulation and therefore serves as a regulator of antigen presentation and other APC functions.

• MeSH
• Adaptor Proteins, Signal Transducing chemistry   metabolism   MeSH
• Lymphocyte Activation MeSH
• Antigen-Presenting Cells immunology   MeSH
• B-Lymphocytes immunology   metabolism   MeSH
• Phosphoproteins metabolism   MeSH
• HEK293 Cells MeSH
• Immunological Synapses chemistry   MeSH
• Humans MeSH
• RNA, Small Interfering MeSH
• Membrane Microdomains chemistry   metabolism   MeSH
• Membrane Proteins chemistry   genetics   immunology   metabolism   MeSH
• Histocompatibility Antigens Class II immunology   metabolism   MeSH
• Mitogen-Activated Protein Kinases metabolism   MeSH
• Molecular Sequence Data MeSH
• Antigen Presentation MeSH
• RNA Interference MeSH
• Amino Acid Sequence MeSH
• Signal Transduction MeSH
• src-Family Kinases metabolism   MeSH
• src Homology Domains MeSH
• T-Lymphocytes immunology   MeSH
• Protein-Tyrosine Kinases chemistry   metabolism   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH