Tetraspanins and Transmembrane Adaptor Proteins As Plasma Membrane Organizers-Mast Cell Case

Abstract

The plasma membrane contains diverse and specialized membrane domains, which include tetraspanin-enriched domains (TEMs) and transmembrane adaptor protein (TRAP)-enriched domains. Recent biophysical, microscopic, and functional studies indicated that TEMs and TRAP-enriched domains are involved in compartmentalization of physicochemical events of such important processes as immunoreceptor signal transduction and chemotaxis. Moreover, there is evidence of a cross-talk between TEMs and TRAP-enriched domains. In this review we discuss the presence and function of such domains and their crosstalk using mast cells as a model. The combined data based on analysis of selected mast cell-expressed tetraspanins [cluster of differentiation (CD)9, CD53, CD63, CD81, CD151)] or TRAPs [linker for activation of T cells (LAT), non-T cell activation linker (NTAL), and phosphoprotein associated with glycosphingolipid-enriched membrane microdomains (PAG)] using knockout mice or specific antibodies point to a diversity within these two families and bring evidence of the important roles of these molecules in signaling events. An example of this diversity is physical separation of two TRAPs, LAT and NTAL, which are in many aspects similar but show plasma membrane location in different microdomains in both non-activated and activated cells. Although our understanding of TEMs and TRAP-enriched domains is far from complete, pharmaceutical applications of the knowledge about these domains are under way.

Keywords: CD9; IgE receptor; LAT; NTAL; membrane microdomains; plasma membrane; signal transduction.

Figure 1

Involvement of TEMs and TRAP-enriched domains in FcεRI signalosome. At the level of the plasma membrane, there is a cross-talk of FcεRI, consisting of four transmembrane subunits (α,β, and γγ), with tetraspanins and TRAPs. These interactions affect FcεRI signal transduction. Phosphorylated ITAMs of FcεRI bind LYN kinase, which is connected to the membrane by palmitoylation and myristoylation. Two important TRAPs, NTAL and LAT, which occupy different nanodomains, are also palmitoylated. Tetraspanins are represented by CD9, which contains six palmitoylation domains and one possible glycosylation site in a small intracellular loop, two disulfide bonds and tetraspanin conserved motif CCG are also depicted in the figure. Finally, integrins are represented by palmitoylated α4β1 that is located in close proximity of CD9. The membrane is connected with the actin cytoskeleton through the FERM (4.1,ezrin/radixin/moesin) domain and actin-binding domain (Abd) of ERM proteins. FERM binds directly to phosphatidylinositol 4 5-bisphosphate (PIP2).

Figure 2

Topography of plasma membrane components in mast cells before and after activation with Ag or CD9-specific antibody. (A) In non-activated cells, tetraspanin CD9 co-localizes with integrin in TEMs. These domains are topographically different from LAT- or NTAL-containing nanodomains, each occupying a distinct plasma membrane region. FcεRI receptor is associated with LYN kinase in another membrane nanodomains. (B) Binding of Ag-specific IgE to FcεRI, followed by exposure to multivalent Ag, causes multimerization of the FcεRI receptors. NTAL and LAT become phosphorylated, but are still separated in different domains and do not co-localize with FcεRI and with each other. In contrast, antibody-mediated aggregation of CD9 brings CD9 into close proximity of NTAL and FcγRs and causes NTAL phosphorylation. Activation through both FcεRI and CD9 leads to dephosphorylation of ERM and dynamic disconnection of the membrane components and actin cytoskeleton. The conclusions indicated above are based on the interpretation of previously published data (Wilson et al., ; Volná et al., ; Hálová et al., 2013).