Association of specific cell-surface glycoproteins with a triton X-100-resistant complex of plasma membrane proteins isolated from T-lymphoma cells (P1798)

Isolation and Analysis of Detergent-Resistant Membrane Fractions

The hypothesis that the Golgi apparatus is capable of sorting proteins and sending them to the plasma membrane through "lipid rafts," membrane lipid domains highly enriched in glycosphingolipids, sphingomyelin, ceramide, and cholesterol, was formulated by van Meer and Simons in 1988 and came to a turning point when it was suggested that lipid rafts could be isolated thanks to their resistance to solubilization by some detergents, namely Triton X-100. An incredible number of papers have described the composition and properties of detergent-resistant membrane fractions. However, the use of this method has also raised the fiercest criticisms. In this chapter, we would like to discuss the most relevant methodological aspects related to the preparation of detergent-resistant membrane fractions, and to discuss the importance of discriminating between what is present on a cell membrane and what we can prepare from cell membranes in a laboratory tube.

Cell-free antibody capture method for analysis of detergent-resistant membrane rafts

Cholesterol-rich microdomains present on the plasma membrane appear to play an important role in spatio-temporal regulation of cell signaling and cell adhesion processes. Compositional heterogeneity of these microdomains and their coalescence during cell-cell interactions may provide one mechanism for triggering and/or regulating signaling cascades from the plasma membrane to the cell interior. Biochemical analyses of distinct lipid microdomain subpopulations and single-rafts obtained from unstimulated and ligand-stimulated cells are critical for deciphering functional role of lipid rafts. We have designed a cell-free assay that captures detergent-resistant lipid rafts with an antibody against a raft-resident molecule and detects the presence of another lipid raft molecule. Moreover, this cell-free assay provides a simple and quick way to examine the simultaneous presence of two proteins in the lipid rafts, and has the potential to estimate trafficking of molecules in and out of the lipid microdomains during cell signaling on a single lipid raft-basis.

Evidence that HLA-G is the functional homolog of mouse Qa-2, the Ped gene product

Qa-2, a murine class Ib major histocompatibility complex (MHC) molecule, is a possible functional homolog of human leukocyte antigen G (HLA-G). Both molecules have been implicated in immunoregulation and embryonic development and both occur in membrane-bound and soluble isoforms that arise by alternative splicing. Soluble splice variants have been implicated in the reproductive functions of HLA-G. While soluble variants of Qa-2 have been previously detected in T lymphocytes, we now demonstrate the presence of mRNA for one of the two known soluble forms of Qa-2 in eight-cell embryos and in blastocysts. Qa-2 is glycosylphosphatidylinositol (GPI) linked in the outer leaflet of the cell membrane and is found in lipid raft microdomains where other raft-associated proteins transduce signals into the cell. In contrast, HLA-G has a truncated six amino acid cytoplasmic tail. By fluorescence co-localization in JEG-3 cells, using fluorescent cholera toxin beta subunit (a lipid raft marker) and anti-HLA-G antibody, we have demonstrated that membrane-bound HLA-G also localizes to lipid rafts, consistent with functional homology between the two molecules. Finally, our experiments in which we have purified Qa-2 and transferred it via a process known as protein painting to Qa-2 negative cells represent a model for potential therapy involving HLA-G.

The state of lipid rafts: from model membranes to cells

Lipid raft microdomains were conceived as part of a mechanism for the intracellular trafficking of lipids and lipid-anchored proteins. The raft hypothesis is based on the behavior of defined lipid mixtures in liposomes and other model membranes. Experiments in these well-characterized systems led to operational definitions for lipid rafts in cell membranes. These definitions, detergent solubility to define components of rafts, and sensitivity to cholesterol deprivation to define raft functions implicated sphingolipid- and cholesterol-rich lipid rafts in many cell functions. Despite extensive work, the basis for raft formation in cell membranes and the size of rafts and their stability are all uncertain. Recent work converges on very small rafts <10 nm in diameter that may enlarge and stabilize when their constituents are cross-linked.

Glycosylphosphatidylinositol-anchored proteins: structure, function, and cleavage by phosphatidylinositol-specific phospholipase C

A wide variety of proteins are tethered by a glycosylphosphatidylinositol (GPI) anchor to the extracellular face of eukaryotic plasma membranes, where they are involved in a number of functions ranging from enzymatic catalysis to adhesion. The exact function of the GPI anchor has been the subject of much speculation. It appears to act as an intracellular signal targeting proteins to the apical surface in polarized cells. GPI-anchored proteins are sorted into sphingolipid- and cholesterol-rich microdomains, known as lipid rafts, before transport to the membrane surface. Their localization in raft microdomains may explain the involvement of this class of proteins in signal transduction processes. Substantial evidence suggests that GPI-anchored proteins may interact closely with the bilayer surface, so that their functions may be modulated by the biophysical properties of the membrane. The presence of the anchor appears to impose conformational restraints, and its removal may alter the catalytic properties and structure of a GPI-anchored protein. Release of GPI-anchored proteins from the cell surface by specific phospholipases may play a key role in regulation of their surface expression and functional properties. Reconstitution of GPI-anchored proteins into bilayers of defined phospholipids provides a powerful tool with which to explore the interactions of these proteins with the membrane and investigate how bilayer properties modulate their structure, function, and cleavage by phospholipases.

The roles of membrane microdomains (rafts) in T cell activation

Detergent-resistant membrane microdomains enriched in sphingolipids, cholesterol and glycosylphosphatidylinositol-anchored proteins play essential roles in T cell receptor (TCR) signaling. These 'membrane rafts' accumulate several cytoplasmic lipid-modified molecules, including Src-family kinases, coreceptors CD4 and CD8 and transmembrane adapters LAT and PAG/Cbp, essential for either initiation or amplification of the signaling process, while most other abundant transmembrane proteins are excluded from these structures. TCRs in various T cell subpopulations may differ in their use of membrane rafts. Membrane rafts also seem to be involved in many other aspects of T cell biology, such as functioning of cytokine and chemokine receptors, adhesion molecules, antigen presentation, establishing cell polarity or interaction with important pathogens. Although the concept of membrane rafts explains several diverse biological phenomena, many basic issues, such as composition, size and heterogeneity, under native conditions, as well as the dynamics of their interactions with TCRs and other immunoreceptors, remain unclear, partially because of technical problems.

A fast, simple and sensitive method for the detection and quantification of detergent-resistant membranes

The aggregation of the T cell receptor and other signaling molecules leads to the formation of large molecular activation clusters in the cell membrane. These molecular clusters are associated with a high concentration of cholesterol, sphingomyelin and gangliosides and were referred to as lipid microdomains. Electron microscopy studies of viable cells indicate that distinct subgroups of lipid microdomains exist and they contain different types of signaling molecules. Lipid microdomains are insoluble in ice-cold solutions containing detergent and are also referred to as detergent-resistant membranes (DRM). Currently, sucrose density centrifugation is the standard method for DRM isolation. Cholera toxin B subunit (CTB) is a specific ligand for ganglioside GM1 and can be used for the detection of GM1 containing DRM. In this paper, we describe a new and simple method for quantification of GM1 associated with DRM. We used a CTB-horseradish peroxidase (HRP) conjugate and 2,2'-azino-di-3-ethyl-benzthiazoline-6-sulfonic acid (ABTS) for the detection of DRM in floating fractions of a sucrose density gradient. Absorbance values (A(405)) were determined using a microtiter plate and an ELISA plate reader. The linear range for the HRP-ABTS reaction was determined in the presence of lysis buffer and sucrose concentrations up to 40%. Linearity of the assay was determined over a wide range (5-1000 microU peroxidase activity per well) in a single experiment and the limit of detection of this method was approximately 10 ng of CTB per gradient fraction. The method is nonradioactive, rapid and easy and can be used for the analysis of DRM resident proteins. We applied this method to Jurkat T cells and after centrifugation observed the existence of DRM floating to 5%/30% sucrose interface. After separation of the sucrose gradient, we identified a large GM1 content in the corresponding fractions 4 to 6. The presence of protein in these fractions was confirmed by silver-stained polyacrylamide gels. We confirmed the presence of adaptor molecules (LAT) and Src kinases (Lck) in the DRM containing fractions 4 to 6.

Relationship of lipid rafts to transient confinement zones detected by single particle tracking

We examined the physical and chemical characteristics of transient confinement zones (TCZs) that are detected in single particle trajectories of molecules moving within the membrane of C3H 10T1/2 murine fibroblasts and their relationship to "rafts." We studied the lateral movement of different membrane molecules thought to partition to varying degrees into or out of the putative lipid domains known as rafts. We found that lipid analogs spend significantly less time in TCZs compared with Thy-1, a glycosylphosphatidylinositol-anchored protein, and GM1, a glycosphingolipid. For Thy-1, we found that zone abundance was markedly reduced by cholesterol extraction, suggesting that a major source of the observed temporary confinement is related to the presence of raft domains. More detailed analysis of particle trajectories reveals that zones can be revisited even tens of seconds after the original escape and that diffusion within the zones is reduced by a factor of approximately 2, consistent with the zone being a cholesterol-rich liquid-ordered phase. Surprisingly, transient confinement was not strongly temperature dependent. Overall, our data demonstrate that there are raft-related domains present in certain regions of the plasma membrane of C3H cells, which can persist for tens of seconds.

Promiscuous antigen presentation by the nonclassical MHC Ib Qa-2 is enabled by a shallow, hydrophobic groove and self-stabilized peptide conformation

BACKGROUND

Qa-2 is a nonclassical MHC Ib antigen, which has been implicated in both innate and adaptive immune responses, as well as embryonic development. Qa-2 has an unusual peptide binding specificity in that it requires two dominant C-terminal anchor residues and is capable of associating with a substantially more diverse array of peptide sequences than other nonclassical MHC.

RESULTS

We have determined the crystal structure, to 2.3 A, of the Q9 gene of murine Qa-2 complexed with a self-peptide derived from the L19 ribosomal protein, which is abundant in the pool of peptides eluted from the Q9 groove. The 9 amino acid peptide is bound high in a shallow, hydrophobic binding groove of Q9, which is missing a C pocket. The peptide makes few specific contacts and exhibits extremely poor shape complementarity to the MHC groove, which facilitates the presentation of a degenerate array of sequences. The L19 peptide is in a centrally bulged conformation that is stabilized by intramolecular interactions from the invariant P7 histidine anchor residue and by a hydrophobic core of preferred secondary anchor residues that have minimal interaction with the MHC.

CONCLUSIONS

Unexpectedly, the preferred secondary peptide residues that exhibit tenuous contact with Q9 contribute significantly to the overall stability of the peptide-MHC complex. The structure of this complex implies a "conformational" selection by Q9 for peptide residues that optimally stabilize the large bulge rather than making intimate contact with the MHC pockets.

Characterization of cholesterol-sphingomyelin domains and their dynamics in bilayer membranes

Lipids segregate with each other into small domains in biological membranes, which can facilitate the associations of particular proteins. The segregation of cholesterol and sphingomyelin (SPM) into domains known as rafts is thought to be especially important. The formation of rafts was studied by using planar bilayer membranes that contained rhodamine-phosphatidylethanolamine (rho-DOPE) as a fluorescent probe, and wide-field fluorescence microscopy was used to detect phase separation of the probe. A fluorescently labeled GM(1), known to preferentially partition into rafts, verified that rho-DOPE faithfully reported the rafts. SPM-cholesterol domains did not form at high temperatures but spontaneously formed when temperature was lowered to below the melting temperature of the SPM. Saturated acyl chains on SPMs therefore promote the formation of rafts. The domains were circular (resolution > or = 0.5 microm), quickly reassumed their circular shape after they were deformed, and merged with each other to create larger domains, all phenomena consistent with liquid-ordered (l(o)) rather than solid-ordered (s(o)) domains. A saturated phosphatidylcholine (PC), disteoryl-PC, could substitute for SPM to complex with cholesterol into a l(o)-domain. But in the presence of cholesterol, a saturated phosphatidylethanolamine or phosphatidylserine yielded s(o)-domains of irregular shape. Lipids with saturated acyl chains can therefore pack well among each other and with cholesterol to form l(o)-domains, but domain formation is dependent on the polar headgroup of the lipid. An individual raft always extended through both monolayers. Degrading cholesterol in one monolayer with cholesterol oxidase first caused the boundary of the raft to become irregular; then the raft gradually disappeared. The fluid nature of rafts, demonstrated in this study, may be important for permitting dynamic interactions between proteins localized within rafts.

Insolubility of lipids in triton X-100: physical origin and relationship to sphingolipid/cholesterol membrane domains (rafts)

The insolubility of lipids in detergents is a useful method for probing the structure of biological membranes. Insolubility in detergents like Triton X-100 is observed in lipid bilayers that exist in physical states in which lipid packing is tight. The Triton X-100-insoluble lipid fraction obtained after detergent extraction of eukaryotic cells is composed of detergent-insoluble membranes rich in sphingolipids and cholesterol. These insoluble membranes appear to arise from sphingolipid- and cholesterol-rich membrane domains (rafts) in the tightly packed liquid ordered state. Because the degree of lipid insolubility depends on the stability of lipid-lipid interactions relative to lipid-detergent interactions, the quantitative relationship between rafts and detergent-insoluble membranes is complex, and can depend on lipid composition, detergent and temperature. Nevertheless, when used conservatively detergent insolubility is an invaluable tool for studying cellular rafts and characterizing their composition.

Signaling through sphingolipid microdomains of the plasma membrane: the concept of signaling platform

Transmembrane signaling requires modular interactions between signaling proteins, phosphorylation or dephosphorylation of the interacting protein partners and temporary elaboration of supramolecular structures, to convey the molecular information from the cell surface to the nucleus. Such signaling complexes at the plasma membrane are instrumental in translating the extracellular cues into intracellular signals for gene activation. In the most straightforward case, ligand binding promotes homodimerization of the transmembrane receptor which facilitates modular interactions between the receptor's cytoplasmic domains and intracellular signaling and adaptor proteins. For example, most growth factor receptors contain a cytoplasmic protein tyrosine kinase (PTK) domain and ligand-mediated receptor dimerization leads to cross phosphorylation of tyrosines in the receptor's cytoplasmic domains, an event that initiates the signaling cascade. In other signaling pathways where the receptors have no intrinsic kinase activity, intracellular nonreceptor PTKs (i.e. Src family PTKs, JAKs) are recruited to the cytoplasmic domain of the engaged receptor. Execution of these initial phosphorylations and their translation into efficient cellular stimulation requires concomitant activation of diverse signaling pathways. Availability of stable, preassembled matrices at the plasma membrane would facilitate scaffolding of a large array of receptors, coreceptors, tyrosine kinases and other signaling and adapter proteins, as it is the case in signaling via the T cell antigen receptor. The concept of the signaling platform has gained usage to characterize the membrane structure where many different membrane-bound components need to be assembled in a coordinated manner to carry out signaling. The structural basis of the signaling platform lies in preferential assembly of certain classes of lipids into distinct physical and functional compartments within the plasma membrane. These membrane microdomains or rafts (Figure 1) serve as privileged sites where receptors and proximal signaling molecules optimally interact. In this review, we shall discuss first how signaling platforms are assembled and how receptors and their signaling machinery could be functionally linked in such structures. The second part of our review will deal with selected examples of raft-based signaling pathways in T lymphocytes and NK cells to illustrate the ways in which rafts may facilitate signaling.

Signal transduction via CD44: role of plasma membrane microdomains

CD44 is the principal cell surface receptor for extracellular matrix glycosaminoglycan hyaluronan. CD44-hyaluronan mediated cell adhesion is important in several pathophysiological processes such as inflammation and metastatic spread of cancer cells. It has been recently recognized that CD44 also functions as a signaling receptor in a variety of cell types. Cell stimulation by monoclonal anti-CD44 antibody or natural CD44 ligands activate several signaling pathways that culminate in cell proliferation, cytokine secretion, chemokine gene expression and cytolytic effector functions. One of the earliest signaling events following stimulation via CD44 is tyrosine phosphorylation of intracellular proteins substrates, and CD44 mediated cellular activation could be abolished by protein tyrosine kinase (PTK) inhibitors. The Src-family non-receptor PTKs such as Lck, Fyn, Lyn and Hck were shown to be coupled to CD44 via sphingolipid-rich microdomains (lipid rafts) of the plasma membrane. Studies on T cell receptor and IgE receptor mediated signaling in lymphocytes and mast cells have consolidated the notion that microdomains consist of signaling platforms where components of multiple signaling pathways are assembled. Co-isolation of CD44 with microdomains strongly suggests that CD44 generates cellular activation signals utilizing the signaling machinery of the plasma membrane microdomains.

Microdomain-dependent regulation of Lck and Fyn protein-tyrosine kinases in T lymphocyte plasma membranes

Src family protein-tyrosine kinases are implicated in signaling via glycosylphosphatidylinositol (GPI)-anchored receptors. Both kinds of molecules reside in opposite leaflets of the same sphingolipid-enriched microdomains in the lymphocyte plasma membrane without making direct contact. Under detergent-free conditions, we isolated a GPI-enriched plasma membrane fraction, also containing transmembrane proteins, selectively associated with sphingolipid microdomains. Nonionic detergents released the transmembrane proteins, yielding core sphingolipid microdomains, limited amounts of which could also be obtained by detergent-free subcellular fractionation. Protein-tyrosine kinase activity in membranes containing both GPI-anchored and transmembrane proteins was much lower than in core sphingolipid microdomains but was strongly reactivated by nonionic detergents. The inhibitory mechanism acting on Lck and Fyn kinases in these membranes was independent of the protein-tyrosine phosphatase CD45 and was characterized as a mixed, noncompetitive one. We propose that in lymphocyte plasma membranes, Lck and Fyn kinases exhibit optimal activity when juxtaposed to the GPI- and sphingolipid-enriched core microdomains but encounter inhibitory conditions in surrounding membrane areas that are rich in glycerophospholipids and contain additional transmembrane proteins.

Detergent-insoluble glycosphingolipid/cholesterol-rich membrane domains, lipid rafts and caveolae (review)

Within the cell membrane glycosphingolipids and cholesterol cluster together in distinct domains or lipid rafts, along with glycosyl-phosphatidylinositol (GPI)-anchored proteins in the outer leaflet and acylated proteins in the inner leaflet of the bilayer. These lipid rafts are characterized by insolubility in detergents such as Triton X-100 at 4 degrees C. Studies on model membrane systems have shown that the clustering of glycosphingolipids and GPI-anchored proteins in lipid rafts is an intrinsic property of the acyl chains of these membrane components, and that detergent extraction does not artefactually induce clustering. Cholesterol is not required for clustering in model membranes but does enhance this process. Single particle tracking, chemical cross-linking, fluorescence resonance energy transfer and immunofluorescence microscopy have been used to directly visualize lipid rafts in membranes. The sizes of the rafts observed in these studies range from 70-370 nm, and depletion of cellular cholesterol levels disrupts the rafts. Caveolae, flask-shaped invaginations of the plasma membrane, that contain the coat protein caveolin, are also enriched in cholesterol and glycosphingolipids. Although caveolae are also insoluble in Triton X-100, more selective isolation procedures indicate that caveolae do not equate with detergent-insoluble lipid rafts. Numerous proteins involved in cell signalling have been identified in caveolae, suggesting that these structures may function as signal transduction centres. Depletion of membrane cholesterol with cholesterol binding drugs or by blocking cellular cholesterol biosynthesis disrupts the formation and function of both lipid rafts and caveolae, indicating that these membrane domains are involved in a range of biological processes.

Effects of cholesterol depletion by cyclodextrin on the sphingolipid microdomains of the plasma membrane

Sphingolipid microdomains are thought to result from the organization of plasma membrane sphingolipids and cholesterol into a liquid ordered phase, wherein the glycosylphosphatidylinositol (GPI)-anchored proteins are enriched. These domains, resistant to extraction by cold Triton X-100, can be isolated as buoyant membrane complexes (detergent-resistant membranes) in isopycnic density gradients. Here the effects of methyl-beta-cyclodextrin (MBCD), a specific cholesterol-binding agent that neither binds nor inserts into the plasma membrane, were investigated on the sphingolipid microdomains of lymphocytes. MBCD released substantial quantities of GPI-anchored Thy-1 and glycosphingolipid GM1, and also other surface proteins including CD45, and intracellular Lck and Fyn kinases. From endothelial cells, MBCD released GPI-anchored CD59, and CD44, but only a negligible amount of caveolin. Most MBCD-released Thy-1 and CD59 were not sedimentable and thus differed from Thy-1 released by membrane-active cholesterol-binding agents such as saponin and streptolysin O, or Triton X-100. Unlike that released by Triton X-100, only part of the Thy-1 molecules released by MBCD was buoyant in density gradients and co-isolated with GM1. Finally, treatment of Triton X-100-isolated detergent-resistant membranes with MBCD extracted most of the cholesterol without affecting the buoyant properties of Thy-1 or GM1. We suggest that (1) MBCD preferentially extracts cholesterol from outside, rather than within the sphingolipid microdomains and (2) this partly solubilizes GPI-anchored and transmembrane proteins from the glycerophospholipid-rich membrane and releases sphingolipid microdomains in both vesicular and non-vesicular form.

Distinct interactions among GPI-anchored, transmembrane and membrane associated intracellular proteins, and sphingolipids in lymphocyte and endothelial cell plasma membranes

Glycosylphosphatidylinositol (GPI)-anchored glycoproteins are enriched in sphingolipid-rich plasma membrane domains, which are often isolated as low-density membrane complexes. This association is believed to arise from the interactions between the GPI-acyl chains and sphingolipids, but is not fully understood. In this study, we compared the physical properties of GPI-anchored glycoproteins from a non-polarized (murine T-lymphocyte) and a polarized (human endothelial) cell by equilibrium density gradient centrifugation after extraction by detergents under identical conditions. Unlike those on epithelial cells, the GPI-anchored proteins of lymphocytes (Thy-1 and the heat stable antigen CD24) were enriched in the floating fractions after extraction over a wide range of octylglucoside concentrations. In contrast, the floatability of endothelial GPI-anchored CD59 was markedly diminished, not only by octylglucoside, but also by increasing concentrations of Triton X-100. Distribution of cholera toxin binding ganglioside GM1 in the sucrose gradient fractions closely followed that of the GPI-anchored proteins in both lymphocytes and endothelial cells under most extraction conditions. Analysis of the intracellular acylated molecules revealed that a significant amount of p56(lck) was always associated with the floating GPI-rich fractions of lymphocytes when extracted by Triton X-100 or octylglucoside at 4 degrees C, while the behaviour of endothelial cell caveolin was comparable to that of CD59. The transmembrane glycoproteins CD45 in lymphocytes and MHC class I antigen in endothelial cells interacted weakly with GPI domains, whereas endothelial CD44 and lymphocyte CD26 displayed a strong association. These results show that: (1) the physical properties of different GPI-anchored proteins may vary significantly; and (2) transmembrane and acylated intracellular proteins could be associated with GPI domains to a variable extent. These differences probably reflect cell type-specific interactions of GPI anchors with the sphingolipid framework of plasma membranes, as well as extracellular interactions of GPI-anchored glycoproteins with neighbouring cell surface molecules.

Identification of detergent-resistant plasma membrane microdomains in dictyostelium: enrichment of signal transduction proteins

Unlike most other cellular proteins, the chemoattractant receptor, cAR1, of Dictyostelium is resistant to extraction by the zwitterionic detergent, CHAPS. We exploited this property to isolate a subcellular fraction highly enriched in cAR1 by flotation of CHAPS lysates of cells in sucrose density gradients. Immunogold electron microscopy studies revealed a homogeneous preparation of membrane bilayer sheets. This preparation, designated CHAPS-insoluble floating fraction (CHIEF), also contained a defined set of 20 other proteins and a single uncharged lipid. Cell surface biotinylation and preembedding immunoelectron microscopy both confirmed the plasma membrane origin of this preparation. The cell surface phosphodiesterase (PDE) and a downstream effector of cAR1, adenylate cyclase (ACA), were specifically localized in these structures, whereas the cell adhesion molecule gp80, most of the major cell surface membrane proteins, cytoskeletal components, the actin-binding integral membrane protein ponticulin, and G-protein alpha- and beta-subunits were absent. Overall, CHIFF represents about 3-5% of cell externally exposed membrane proteins. All of these results indicate that CHIFF is derived from specialized microdomains of the plasma membrane. The method of isolation is analogous to that of caveolae. However, we were unable to detect distinct caveolae-like structures on the cell surface associated with cAR1, which showed a diffuse staining profile. The discovery of CHIFF facilitates the purification of cAR1 and related signaling proteins and the biochemical characterization of receptor-mediated processes such as G-protein activation and desensitization. It also has important implications for the "fluid mosaic" model of the plasma membrane structures.

Thy-3, a developmentally regulated T-cell glycoprotein associated to Thy-1 in detergent-resistant membrane microdomains

The function of the Thy-1 molecule, a major T-cell glycoprotein, is still obscure. Its functional properties might be due to the anchoring via a glycosylphosphatidylinositol group which favors gathering of molecules in functional membrane subregions called microdomains. Using novel monoclonal antibodies, we describe a 53-kDa Thy-1-associated glycoprotein called Thy-3. Thy-3 expression is restricted to T lymphocytes, becomes detectable on double-positive thymocytes, and depends on that of Thy-1. Anti-Thy-3 antibodies immunoprecipitate Thy-1 and Thy-3 or Thy-3 alone in detergents which preserve or disrupt microdomains, respectively. These antibodies induce thymocyte aggregation and interfere with adhesion of thymocytes to a thymic epithelial cell line as previously shown with anti-Thy-1 antibodies. Thus, Thy-3 is a T lineage-specific glycoprotein associated to Thy-1 in membrane microdomains and might contribute to the function of Thy-1 in T-cell differentiation.

Differential targeting of T- and N-cadherin in polarized epithelial cells

To test whether glycosyl phosphatidylinositol-linked T-cadherin is a component of cell junctions like classical cadherins, we have examined its distribution and targeting in polarized epithelial cells. In vivo, T-cadherin was detected on the apical cell surface of the chick intestinal epithelium. In cultures of transfected Madin-Darby canine kidney cells, T-cadherin was also expressed apically, whereas classical N-cadherin resided basolaterally. Both cadherins were directly targeted to their respective membrane domains. Mutant proteins were expressed in Madin-Darby canine kidney cells to identify the regions responsible for differential cadherin localization. NDeltacyt, an N-cadherin cytoplasmic domain deletion mutant, was stably distributed basolaterally. This mutant was transported to both the apical and basolateral membrane compartments, followed by preferential removal from the apical surface. T-NDeltacyt, a T-cadherin mutant with the N-cadherin cytoplasmic domain deletion, was localized basolaterally, whereas N-TGPI, a GPI-anchored N-cadherin mutant, resided at the apical domain. The T-cadherin carboxyl-terminal 76 amino acids contain the apical targeting signal and include the signal for GPI anchor attachment. Basolateral localization of N-cadherin is achieved through targeting signals in the cytoplasmic domain. Thus, GPI-linked T-cadherin is not a component of cell junctions, consistent with a function as a recognition rather than a cell adhesion molecule.

Noncovalent associations of T lymphocyte surface proteins

A number of T cell surface transmembrane molecules such as CD2, CD4, CD8, lymphocyte functional antigen (LFA)-1 and CD45 are known to interact functionally with the T cell receptor (TCR) complex during T cell activation. Several previous communications have also reported physical associations between some of these molecules. On the other hand, there are indications that signaling through T cell surface molecules anchored via glycosylphosphatidylinositol (GPI), such as Thy-1, Ly-6 or CD59, is dependent on the TCR. Therefore, it was of interest to determine in a systematic way which T cell surface molecules are noncovalently associated with the TCR/CD3 complex and with the major intracellular signaling molecules, the protein tyrosine kinases of the Src family. To this aim, membrane proteins of human thymoma HPB-ALL cells were solubilized in a solution of the mild detergent Brij-58 and subjected to immunoprecipitation followed by in vitro kinase assays. Two types of large complexes containing protein tyrosine kinases were observed: the first one contained CD3 and the transmembrane proteins CD2, CD4, CD5, CD6, CD7, CD8, CD11a, CD38, CD43, CD45, CD71, CD98 and CD99 and the other contained mainly the GPI-anchored proteins CD48, CD55, CD59 and CDw108 as well as a fraction of CD4 and CD8. The GPI-anchored protein complexes were of larger size and lower buoyant density than the CD3 complexes. In agreement with these biochemical data, co-cross-linking of CD3 with most of the relevant transmembrane proteins on the surface of another T cell line, Jurkat, markedly enhanced tyrosine phosphorylation of several intracellular proteins. These data indicate the existence of at least two types of membrane microdomains of very different composition in the membranes of T cells which may play a role in signaling through different types of receptors and in functional cooperation between TCR/CD3 and various accessory molecules.

Separation of caveolae from associated microdomains of GPI-anchored proteins

In situ coating of the surface of endothelial cells in rat lung with cationic colloidal silica particles was used to separate caveolae from detergent-insoluble membranes rich in glycosyl phosphatidylinositol (GPI)-anchored proteins but devoid of caveolin. Immunogold electron microscopy showed that ganglioside GM1-enriched caveolae associated with an annular plasmalemmal domain enriched in GPI-anchored proteins. The purified caveolae contained molecular components required for regulated transport, including various lipid-anchored signaling molecules. Such specialized distinct microdomains may exist separately or together in the plasma membrane to organize signaling molecules and to process surface-bound ligands differentially.

VIP21-caveolin, a membrane protein constituent of the caveolar coat, oligomerizes in vivo and in vitro

VIP21-caveolin is a membrane protein, proposed to be a component of the striated coat covering the cytoplasmic surface of caveolae. To investigate the biochemical composition of the caveolar coat, we used our previous observation that VIP21-caveolin is present in large complexes and insoluble in the detergents CHAPS or Triton X-114. The mild treatment of these insoluble structures with sodium dodecyl sulfate leads to the detection of high molecular mass complexes of approximately 200, 400, and 600 kDa. The 400-kDa complex purified to homogeneity from dog lung is shown to consist exclusive of the two isoforms of VIP21-caveolin. Pulse-chase experiments indicate that the oligomers form early after the protein is synthesized in the endoplasmic reticulum (ER). VIP21-caveolin does indeed insert into the ER membrane through the classical translocation machinery. Its hydrophobic domain adopts an unusual loop configuration exposing the N- and C-flanking regions to the cytoplasm. Similar high molecular mass complexes can be produced from the in vitro-synthesized VIP21-caveolin. The complex formation occurs only if VIP21-caveolin isoforms are properly inserted into the membrane; formation is cytosol-dependent and does not involve a vesicle fusion step. We propose that high molecular mass oligomers of VIP21-caveolin represent the basic units forming the caveolar coat. They are formed in the ER and later, between the ER and the plasma membrane, these oligomers could associate into larger detergent-insoluble structures.

Solubilization of glycosyl-phosphatidylinositol-anchored proteins in quiescent and stimulated neutrophils

In human neutrophils, alkaline phosphatase (AlkPase), a low-affinity receptor for IgG (FcRIIIB), and complement decay accelerating factor (DAF) are glycosyl-phosphatidylinositol (GPI)-anchored proteins. Varying greatly in biological function these three integral membrane proteins exhibit regulated cell surface expression in neutrophils. Defined by their common membrane-linkage motif, AlkPase, FcRIIIB, and DAF can be released from the lipid bilayer by the action of phosphatidylinositol-specific phospholipase C and are relatively resistant to low temperature extraction with Triton X-100 (TX-100). In this study we show that neutrophil AlkPase, FcRIII, and DAF display differential extractibility; they are relatively insensitive to TX-100 solubilization at 4 degrees C, but are readily extracted with TX-100 at 37 degrees C or by the detergent octyl glucoside at 4 degrees C. The differential extractibility of these GPI-anchored proteins is the same in unstimulated cells, where these proteins exist primarily in an intracellular pool, and stimulated cells, where they are expressed principally at the cell surface. However, no differential extraction effect is observed with two neutrophil transmembrane proteins, complement receptor 1 (CD35, CR1) and MHC Class I in either stimulated or unstimulated cells.

Both sphingolipids and cholesterol participate in the detergent insolubility of alkaline phosphatase, a glycosylphosphatidylinositol-anchored protein, in mammalian membranes

SPB-1, a Chinese hamster ovary cell variant defective in serine palmitoyltransferase activity for sphingolipid synthesis, provides a useful system for studying the effects of sphingolipids and/or cholesterol deprivation on cellular functions and membrane properties. To investigate whether there was an interaction among sphingolipids, cholesterol, and glycosylphosphatidylinositol (GPI)-anchored proteins in biological membranes, we introduced human placental alkaline phosphatase (PLAP) in SPB-1 and in wild type cells by stable transfection and examined the effects of sphingolipid and/or cholesterol deprivation on the solubility of PLAP in Triton X-100. Although the PLAP solubility of the membranes isolated from the control cells in Triton X-100 was only 10%, deprivation of sphingolipid and cholesterol further enhanced the solubility, which reached 50% when both sphingolipids and cholesterol were deprived. The enhanced solubility was suppressed to the control level by metabolic complementation with exogenous sphingosine and cholesterol. The sphingolipid and cholesterol content of the isolated membranes changed independently, eliminating the possibility that sphingolipid deprivation induced a reduction in cellular cholesterol and enhanced PLAP solubility and vice versa. It was also unlikely that the enhanced solubility was due to structural changes in PLAP molecules since, regardless of sphingolipid and cholesterol deprivations, almost all PLAP had the GPI-anchor moiety and there were no differences in the apparent molecular weight of the protein in supernatant and precipitate fractions of the detergent-treated membranes. In addition, the expression level of caveolin in the isolated membranes was not significantly affected by sphingolipids and/or cholesterol depletion. These results indicated that both sphingolipids and cholesterol were involved in the PLAP insolubility and suggested that these lipids coordinately played a role in formation of Triton X-100-resistant complexes.

Caveolae from luminal plasmalemma of rat lung endothelium: microdomains enriched in caveolin, Ca(2+)-ATPase, and inositol trisphosphate receptor

A distinctive feature of many endothelia is an abundant population of noncoated plasmalemmal vesicles, or caveolae. Caveolae have been implicated in many important cellular processes, including transcytosis, endocytosis, potocytosis, and even signal transduction. Because caveolae have not been purified from endothelial cell surfaces, little is known directly about their structure and function in the endothelium. To delineate the transport role of these caveolae, we purified them from isolated luminal endothelial plasma membranes of rat lung. The rat lung luminal endothelial cell surfaces were isolated after coating them, in situ, with positively charged colloidal silica. The caveolae were then separated from these coated membranes and purified to yield a homogeneous population of morphologically distinct vesicles enriched in the structural protein caveolin. As with caveolae found on the endothelial cell surface in vivo, these highly purified caveolae contained the plasmalemmal Ca(2+)-ATPase and inositol 1,4,5-trisphosphate surface receptors. By contrast, other plasma membrane proteins were excluded from the caveolae, including angiotensin-converting enzyme, beta-actin, and band 4.1. The purified caveolae appeared to represent specific microdomains of the cell surface with their own unique molecular topography.

VIP21-Caveolin, a protein of the trans-Golgi network and caveolae

VIP21-Caveolin is a component of the filamentous coat surrounding the invaginations of the plasma membrane called caveolae. Unlike the vesicular coat proteins identified so far, VIP21-Caveolin can be classified as an integral membrane protein. Furthermore, it is found in high molecular mass oligomers. Based on its localisation in specialised membrane subdomains, a role for VIP21-Caveolin in membrane protein sorting has been proposed.

Caveolin forms a hetero-oligomeric protein complex that interacts with an apical GPI-linked protein: implications for the biogenesis of caveolae

Glycosyl-phosphatidylinositol (GPI)-linked proteins are transported to the apical surface of epithelial cells where they undergo cholesterol-dependent clustering in membrane micro-invaginations, termed caveolae or plasmalemmal vesicles. However, the sorting machinery responsible for this caveolar-clustering mechanism remains unknown. Using transfected MDCK cells as a model system, we have identified a complex of cell surface molecules (80, 50, 40, 22-24, and 14 kD) that interact in a pH- and cholesterol-dependent fashion with an apical recombinant GPI-linked protein. A major component of this hetero-oligomeric protein complex is caveolin, a type II transmembrane protein. As this hetero-oligomeric caveolin complex is detectable almost immediately after caveolin synthesis, our results suggest that caveolae may assemble intracellularly during transport to the cell surface. As such, our studies have implications for understanding both the intracellular biogenesis of caveolae and their subsequent interactions with GPI-linked proteins in epithelia and other cell types.

The association of the protein tyrosine kinases p56lck and p60fyn with the glycosyl phosphatidylinositol-anchored proteins Thy-1 and CD48 in rat thymocytes is dependent on the state of cellular activation

Cell surface glycoproteins anchored to the plasma membrane via glycosylphosphatidylinositol (GPI) structures, and hence having no cytoplasmic domains, can nevertheless transmit activation signals in lymphocytes. By immunoprecipitation from detergent lysates and in vitro immune complex kinase reactions the GPI-anchored molecules Thy-1 and CD48 are shown to be associated with multimolecular complexes of phosphoproteins including the protein tyrosine kinases p56lck and p60fyn in both rat and mouse thymocytes. Moreover, the kinase activity associated with Thy-1 on rat thymocytes is shown to be dependent on the activation state of the cells, with stimulation by the lectin, concanavalin A, producing a marked decrease in Thy-1-associated kinase activity. In such activated cells, there is an increased association of kinase activity with CD48, but this may be explained in terms of increased surface expression of CD48 and of increased total kinase activity. Additional phosphoproteins of 85, 36 and 32 kDa were consistently seen as components of the complexes.

Signal transducing molecules and glycosyl-phosphatidylinositol-linked proteins form a caveolin-rich insoluble complex in MDCK cells

GPI-linked protein molecules become Triton-insoluble during polarized sorting to the apical cell surface of epithelial cells. These insoluble complexes, enriched in cholesterol, glycolipids, and GPI-linked proteins, have been isolated by flotation on sucrose density gradients and are thought to contain the putative GPI-sorting machinery. As the cellular origin and molecular protein components of this complex remain unknown, we have begun to characterize these low-density insoluble complexes isolated from MDCK cells. We find that these complexes, which represent 0.4-0.8% of the plasma membrane, ultrastructurally resemble caveolae and are over 150-fold enriched in a model GPI-anchored protein and caveolin, a caveolar marker protein. However, they exclude many other plasma membrane associated molecules and organelle-specific marker enzymes, suggesting that they represent microdomains of the plasma membrane. In addition to caveolin, these insoluble complexes contain a subset of hydrophobic plasma membrane proteins and cytoplasmically-oriented signaling molecules, including: (a) GTP-binding proteins--both small and heterotrimeric; (b) annex II--an apical calcium-regulated phospholipid binding protein with a demonstrated role in exocytic fusion events; (c) c-Yes--an apically localized member of the Src family of non-receptor type protein-tyrosine kinases; and (d) an unidentified serine-kinase activity. As we demonstrate that caveolin is both a transmembrane molecule and a major phospho-acceptor component of these complexes, we propose that caveolin could function as a transmembrane adaptor molecule that couples luminal GPI-linked proteins with cytoplasmically oriented signaling molecules during GPI-membrane trafficking or GPI-mediated signal transduction events. In addition, our results have implications for understanding v-Src transformation and the actions of cholera and pertussis toxins on hetero-trimeric G proteins.

Thy-1 glycoprotein and src-like protein-tyrosine kinase p53/p56lyn are associated in large detergent-resistant complexes in rat basophilic leukemia cells

Thy-1 is a surface glycoprotein that is attached to the plasma membrane by a glycosyl-phosphatidyl-inositol anchor. Crosslinking of Thy-1 in rat mast cells and basophilic leukemia cells (RBL-2H3) induces cell activation including histamine release and tyrosine phosphorylation of several proteins. Here we show that glycosyl-phosphatidylinositol-linked Thy-1 forms noncovalent complexes with src-related protein-tyrosine kinase p53/p56lyn and other protein-tyrosine kinases and/or their substrates. These complexes are resistant to solubilization by a nonionic detergent, sedimentable at 200,000 x g, and very large ( > 10 MDa) as determined by gel chromatography. Activation of RBL-2H3 cells by crosslinking of the high-affinity IgE receptors resulted in decreased recovery of the complexes. The combined data indicate the existence of large detergent-resistant domains in the surface membrane of mast cells that may play an important role in their activation.

Spontaneous incorporation of the glycosyl-phosphatidylinositol-linked protein Thy-1 into cell membranes

Thy-1 is a membrane protein that is attached to the plasma membrane by a glycosyl-phosphatidylinositol anchor. Purified rat brain Thy-1 could be reincorporated into the plasma membrane of murine Thy-1- cells directly from aqueous suspension and without the use of detergents. A peripheral staining pattern similar to that observed for endogenous Thy-1 was achieved. Treatment with phosphatidylinositol-specific phospholipase C removed nearly all antibody staining due to either endogenous or inserted Thy-1. Fluorescence recovery after photobleaching (FRAP) was used to compare the lateral mobility of endogenous and inserted Thy-1. Both forms exhibited large lateral diffusion coefficients, but with a substantial immobile fraction (approximately 50%) indicating that the immobile fraction was not due either to chemical differences between inserted and native Thy-1 or to some surface Thy-1 molecules having a protein anchor. However, the inserted Thy-1 failed to activate mouse T lymphocytes upon crosslinking as assayed by [3H]thymidine uptake. Since Thy-1 could be directly labeled with rhodamine, the effect of the size of the labeling ligand on the mobility obtained by the FRAP technique could be explored. Rhodamine-conjugated MRC-OX7 monoclonal antibody or its fragments [R-F(ab)2 or R-Fab] were compared with rhodamine as labels for Thy-1. The measured diffusion coefficients were 1.6 x 10(-9), 2.0 x 10(-9), and 3.2 x 10(-9) cm2/sec for Thy-1 labeled with R-F(ab)2, R-Fab, and rhodamine, respectively; mobile fractions were all in the 40-50% range. Thus, the size of the ligand affects the lateral mobility of this labeled membrane protein to a measurable extent.

Sorting of GPI-anchored proteins to glycolipid-enriched membrane subdomains during transport to the apical cell surface

We show that a protein with a glycosylphosphatidyl inositol (GPI) anchor can be recovered from lysates of epithelial cells in a low density, detergent-insoluble form. Under these conditions, the protein is associated with detergent-resistant sheets and vesicles that contain other GPI-anchored proteins and are enriched in glycosphingolipids, but do not contain a basolateral marker protein. The protein is recovered in this complex only after it has been transported to the Golgi complex, suggesting that protein-sphingolipid microdomains form in the Golgi apparatus and plasma membrane and supporting the model proposed by Simons and colleagues for sorting of certain membrane proteins to the apical surface after intracellular association with glycosphingolipids.

Phosphatidylinositol membrane anchors and T-cell activation

A number of lymphocyte proteins are attached to the cell membrane via glycophosphatidylinositol-anchoring domains. Antibodies specific for several of these proteins are potent mitogens for T cells. In this article, Peter Robinson reviews recent evidence for the involvement of these membrane anchors in cell signalling events and discusses their possible significance in providing antigen-independent costimulatory signals to lymphoid cells.

Isolation of high-affinity murine interleukin 2 receptors as detergent-resistant membrane complexes

Murine T cells and T cell lines bearing high- and low-affinity receptors for interleukin (IL) 2 were chemically cross-linked to radiolabeled IL 2 and subjected to differential detergent extractions to evaluate the extent of IL 2 receptor association with the nonionic detergent-resistant framework of the plasma membrane. Low-affinity receptors were readily solubilized by nonionic detergent extraction of whole cross-linked cells, while solubilization of high-affinity receptors required a stronger ionic detergent suggesting their association with a membrane structure that is resistant to nonionic detergents. To achieve physical separation of low- and high-affinity receptors, cells cross-linked to 125I-labeled IL 2 were centrifuged through a sucrose barrier containing Triton X-100. Alternatively, Triton X-114 extracts of plasma membrane fractions were partitioned into aqueous and detergent phases. By either approach, high-affinity receptors differed from low-affinity ones by their increased density and consisted of detergent-resistant complexes containing p55-p75 heterodimers. The low-affinity receptors, on the contrary, were of low density and consisted exclusively of detergent-soluble p55 subunits. High density and resistance to nonionic detergent extraction of high-affinity IL 2 receptors suggest their integration into lateral microdomains of the detergent-resistant framework of the plasma membrane.

Differential release of cellular and scrapie prion proteins from cellular membranes by phosphatidylinositol-specific phospholipase C

The abnormal isoform of the scrapie prion protein PrPSc is both a host-derived protein and a component of the infectious agent causing scrapie. PrPSc and the normal cellular isoform PrPC have different physical properties that apparently arise from a posttranslational event. Both PrP isoforms are covalently modified at the carboxy terminus by a glycoinositol phospholipid. Using preparations of dissociated cells derived from normal and scrapie-infected hamster brain tissue, we find that the majority of PrPC is released from membranes by phosphatidylinositol-specific phospholipase C (PIPLC), while PrPSc is resistant to release. In contrast, purified denatured PrP 27-30 (which is formed from PrPSc during purification by proteolysis of the amino terminus) is completely cleaved by PIPLC. Incubation of the cell preparations with proteinase K cleaves PrPSc to form PrP 27-30, demonstrating that PrPSc is accessible to added enzymes. We have also developed a protocol involving biotinylation that gives a quantitative estimate of the fraction of a protein exposed to the cell exterior. Using this strategy, we find that a large portion of PrPSc in the cell preparations reacts with a membrane-impermeant biotinylation reagent. Whether alternative membrane anchoring of PrPSc, inaccessibility of the glycoinositol phospholipid anchor to PIPLC, or binding to another cellular component is responsible for the differential release of prion proteins from cells remains to be determined.

B cell-stimulating activity of lymphoid cell membrane fractions

We had previously found that a mutagenized subline of the mouse thymoma EL4 very efficiently stimulates B cells via direct cell-cell contact, thereby inducing the responsiveness of B cells to cytokines. In the present study, we investigated whether this effect could also be mediated by plasma membranes of EL4 (and other) cells. By equilibrium centrifugation of cell homogenates, four cell membrane fractions of different densities were obtained. These were tested for (a) stimulation of B cell proliferation in conjunction with EL4 supernatant as source of cytokines, and (b) enhancement of B cell proliferation at suboptimal concentration of lipopolysaccharide. It turned out that all membrane fractions from a variety of T lineage cells (mutant EL4, parent EL4, BW5147, P198 thymomas, normal T cells) and B lineage cells (BCL1 lymphoma, X63Ag8 cytoplasma, normal B cells) exhibited similar B cell stimulating activity in both assays. Interleukin 1 activity was not detected in the membrane fractions. Heat treatment abolished all activity showing that protein at least was involved. Either protease treatment or extraction with detergent abolished the activity of subcellular fractions rich in intracellular membranes but not that of fractions most enriched in surface membranes. Finally, erythrocyte membranes also displayed B cell-stimulating activity sensitive to protease and detergent extraction. In contrast, and in confirmation of a previous study, liver cell membrane was inhibitory in the B cell proliferation assay with lipopolysaccharide. In conclusion, the effects of cell membranes did not reflect the unique activity of intact mutant EL4 cells. However, with respect to our data it is conceivable that membrane proteins with relatively nonspecific activity and wide distribution among lymphoid cells could play a role in T cell help together with molecules specialized in cell adhesion and cell triggering.

Anchoring mechanisms for LFA-3 cell adhesion glycoprotein at membrane surface

The manner in which a membrane protein is anchored to the lipid bilayer may have a profound influence on its function. Most cell surface membrane proteins are anchored by a membrane-spanning segment(s) of the polypeptide chain, but another type of anchor has been described for several proteins: a phosphatidyl inositol glycan moiety, attached to the protein C terminus. This type of linkage has been identified on membrane proteins involved in adhesion and transmembrane signalling and could be important in the execution of these functions. We report here that an immunologically important adhesion glycoprotein, lymphocyte function-associated antigen 3 (LFA-3), can be anchored to the membrane by both types of mechanism. These two distinct cell-surface forms of LFA-3 are derived from different biosynthetic precursors. The existence of a phosphatidyl-inositol-linked and a transmembrane anchored form of LFA-3 has important implications for adhesion and transmembrane signalling by LFA-3.

The Thy-1 antigen exhibits rapid lateral diffusion in the plasma membrane of rodent lymphoid cells and fibroblasts

Thy-1 is a plasma membrane protein, but its primary structure lacks the typical membrane-spanning sequence. Recent studies revealed that a glycophospholipid is covalently bound to the carboxyl terminus, suggesting that the protein is integrated into the plasma membrane by this lipid moiety. Lateral diffusion of Thy-1 was measured in mouse thymocytes, lymphoma cells, and fibroblasts by the fluorescence recovery after photobleaching technique. Thy-1 was labeled with rhodamine-conjugated anti-Thy-1 monoclonal antibodies. Diffusion coefficients of 2-4 X 10(-9) cm2/sec were obtained for the antigen-antibody complex in all the cell types. About 50% of the Thy-1 was mobile. The diffusion coefficient for the mobile fraction of Thy-1 is considerably larger than the diffusion coefficients of many other plasma membrane proteins. Rather, the diffusion coefficient of Thy-1 is similar to those of lipid analogs embedded in the same membrane, providing strong support for the suggested lipid anchoring of this antigen.

Agorins: major structural proteins of the plasma membrane skeleton of P815 tumor cells

Plasma membranes of P815 mastocytoma cells contain a set of proteins that remain selectively insoluble upon extraction of the membranes with Triton X-100, and appear to form a membrane skeletal matrix independent of the filamentous cytoskeletal systems. EGTA treatment of the matrix was found to release approximately 25% of the protein as polypeptides of 70, 69, 38, and 36 kD, all of which appear to be peripheral components associated with the cytoplasmic face of the plasma membrane via divalent cation-dependent interactions. About 75% of the total matrix protein was recovered in the EGTA-insoluble fraction. Actin accounted for approximately 5% of the total protein in the EGTA-insoluble fraction. The rest was accounted for by two novel proteins of 20 and 40 kD which, despite their relatively low molecular weights, do not enter SDS PAGE gels. Together these proteins account for approximately 15% of the total plasma membrane protein, and are thus present in much higher amounts than any other characterized protein of nucleated cell plasma membranes. Based on the extensive associations of these proteins to form very large detergent-insoluble structures, we propose that they may be named agorin I, the 20-kD protein, and agorin II, the 40-kD protein, from the Greek agora meaning assembly. The amount and properties of these proteins and the appearance of the EGTA-insoluble material in thin-section electron micrographs indicate that the agorins are the major structural elements of the membrane matrix, and thus of the putative membrane skeleton.

Qa gene expression: biosynthesis and secretion of Qa-2 molecules in activated T cells

The biosynthesis and expression of the tissue-specific class I molecule Qa-2 have been studied in resting and activated T-cell populations. Polyclonal activation of T lymphocytes induces a 3- to 4-fold increase in the biosynthesis of Qa-2 molecules but no increase in cell-surface levels. Analysis of the biosynthetic pathway of the Qa-2 molecule in activated lymphocytes reveals that approximately equal to 70% of the newly synthesized Qa-2 molecules are secreted as soluble molecules. In resting-cell populations, Qa-2 remains entirely cell-associated. This process is unique to the Qa-2 molecule, since other class I molecules (e.g., H-2Kb and H-2Db) synthesized by activated cells remain cell-associated. The possibility that the secreted Qa-2 molecule is the product of a new Qa gene or an alternatively spliced mRNA is considered. These results indicate that the Qa-2 molecules may not just function as a cell-surface recognition structure but also may serve a role as a soluble factor synthesized by activated lymphoid cell populations.