A New Type of Membrane Raft-Like Microdomains and Their Possible Involvement in TCR Signaling

Abstract

Membrane rafts and signaling molecules associated with them are thought to play important roles in immunoreceptor signaling. Rafts differ in their lipid and protein compositions from the rest of the membrane and are relatively resistant to solubilization by Triton X-100 or similar detergents, producing buoyant, detergent-resistant membranes (DRMs) that can be isolated by density gradient ultracentrifugation. One of the key signaling molecules present in T cell DRMs is the transmembrane adaptor protein LAT (linker for activation of T cells). In contrast to previous results, a recent study demonstrated that a LAT construct not present in the buoyant DRMs is fully able to support TCR signaling and development of T cells in vivo. This finding caused doubts about the real physiological role of rafts in TCR signaling. In this study, we demonstrate that these results can be explained by the existence of a novel type of membrane raft-like microdomains, producing upon detergent solubilization “heavy DRMs” containing a number of membrane molecules. At a moderate level of expression, LAT supported TCR signaling more efficiently than constructs targeted to the microdomains producing heavy DRMs or to nonraft membrane. We suggest that different types of membrane microdomains provide environments regulating the functional efficiencies of signaling molecules present therein.

Dissecting Mannose 6-Phosphate-Insulin-like Growth Factor 2 Receptor Complexes That Control Activation and Uptake of Plasminogen in Cells

“…Likewise in the control cells, the majorities of full-length uPAR together with uPA and ␤3 integrin molecules were seen in the heavy density fractions corresponding to detergent-soluble proteins. Alternatively, they could also be part of the recently described heavy rafts (35).…”

Raft domains of variable properties and compositions in plasma membrane vesicles

“…23), the PMs of many nonpolarized cells may be expected to have small, short-lived domains that are only observable by indirect methods (20). Similarly, the relatively disordered raft domains observed here (e.g., in NEM GPMVs) could be the physicochemical analogs of various "nonraft" PM domains inferred from biochemical and nanoscopic observations, including GPI-anchored protein (AP) domains (24), heavy rafts (25), Lubrol rafts (26), Hras domains (27), T-cell receptor islands (28), and tetraspanin webs (29). Thus, the raft principle-defined as preferential interactions between sterols, saturated lipids, glycosphingolipids, and certain proteins-could be employed by the cell to produce highly ordered, stable domains containing only lipids and lipid-anchored proteins (like GPI-AP domains inferred from FRET experiments or Ras domains observed by EM) as well as more disordered, TM protein-inclusive domains that need not necessarily be insoluble in nonionic detergents.…”

A specific type of membrane microdomains is involved in the maintenance and translocation of kinase active Lck to lipid rafts

“…Light DRMs, corresponding to classical LR, fractionate as low density, light fractions by discontinued sucrose density gradient centrifugation (SDGC) [15]. Notwithstanding controversies surrounding the existence of LR in an in vivo setting [16,17], including the question to what extent DRMs correspond to bona-fide native LR [18], recent experiments targeting critical signaling molecules to distinct membrane compartments [19][20][21][22], together with electron, fluorescent microscopy and nanoscopy (reviewed [23]) support the notion of a critical role of LR in the initiation of T cell signaling. Recently, a distinct type of microdomains, called high-density or heavy DRMs, was proposed to be involved in signaling in immune cells [21,22,24].…”