Structure of a specific peptide complex of the carboxy-terminal SH2 domain from the p85 alpha subunit of phosphatidylinositol 3-kinase.

We have determined the solution structure of the C-terminal SH2 domain of the p85 alpha subunit of human phosphatidylinositol (PI) 3-kinase (EC 2.7.1.137) in complex with a phosphorylated tyrosine pentapeptide sequence from the platelet-derived growth factor receptor using heteronuclear nuclear magnetic resonance spectroscopy. Overall, the structure is similar to other SH2 domain complexes, but displays different detail interactions within the phosphotyrosine binding site and in the recognition site for the +3 methionine residue of the peptide, the side chain of which inserts into a particularly deep and narrow pocket which is displaced relative to that of other SH2 domains. The contacts made within this +3 pocket provide the structural basis for the strong selection for methionine at this position which characterizes the SH2 domains of PI3-kinase. Comparison with spectral and structural features of the uncomplexed domain shows that the long BG loop becomes less mobile in the presence of the bound peptide. In contrast, extreme resonance broadening encountered for most residues in the beta D', beta E and beta F strands and associated connecting loops of the domain in the absence of peptide persists in the complex, implying conformational averaging in this part of the molecule on a microsecond-to-millisecond time scale.

Direct modification of the glycocalyx of a cultured muscle cell line by incorporation of foreign gangliosides and an integral membrane glycoprotein.

As part of a program to better understand the cause-or-effect nature of the relationship between cell surface carbohydrate and cell properties and behaviour, experiments have been carried out on direct modification of the glycocalyx of cultured cells. Modification was by incorporation of gangliosides and an integral membrane glycoprotein chosen to be dissimilar to species occurring naturally in the cell line. Two methods of incorporation were investigated: simple addition of the new components to the culture medium for various times, or assembly of the components into the walls of lipid vesicles which were subsequently fused with cells. Gangliosides from beef brain and glycophorin, the major human erythrocyte sialoglycoprotein, were successfully added to the surface of myoblasts in quantities sufficient to represent a significant perturbation. Changes in cell adhesion, morphology, and viability were observed which seem to be a direct result of glycocalyx modification.

Glycophorin and the concanavalin A receptor of human erythrocytes: their receptor function in lipid bilayers.

Two integral glycoproteins from the human erythrocyte have been studied after their incorporation into lipid bilayer systems. Glycophorin (which is the M/N blood group determinant) and the concanavalin A receptor were isolated and purified prior to incorporation into model membranes by dialytic removal of detergent from lipid/protein solutions. Under the conditions described, glycoprotein receptors maintain their function in that they bind external agents specific for them, such as concanavalin A and immunoglobulins. So-called intramembranous particles are a feature of freeze-fractured preparations of lipid bilayers containing either (or both) glycoprotein(s), and to some extent each has a characteristic particle appearance. Liposomes containing the concanavalin A receptor (with or without glycophorin) are agglutinable by concanavalin A, whereas human erythrocytes are normally considered to be nonagglutinable by this lectin. Liposomes containing glycophorin alone are readily agglutinable by the appropriate glycophorin-directed M/N antiserum, as are human erythrocytes. The added presence of concanavalin A receptor in the liposomes can markedly inhibit agglutination by M/N antiserum without preventing immunoglobulin binding.

Isolation and incorporation into lipid vesicles of a concanavalin A receptor from human erythrocytes.

Affinity chromatography has been used to isolate a concanavalin A receptor portion of Band 3 from humen erythrocytes in the presence of the readily-dialysable detergent, dodecyltrimethylammonium bromide. Addition of phospholipids to the isolated fraction and removal of detergent by dialysis leads to formation of vesicles containing the receptor. Intramembranous particles similar in size and shape to those seen in intact erythrocytes are a characteristic of the reconstituted preparations. Vesicles containing receptor bind concanavalin A with high affinity.

Glycolipids in model membranes. Spin label and freeze-etch studies.

1. Several types of glycolipid are examined in lipid bilayer model membranes as part of a program to clarify their fuction in living cells. 2. Data obtained with three spin labelled derivatives of galactosyl ceramide is reported showing a fatty acid fluidity gradient similar to that obtained with phospholipid spin labels. Some possible structural implications of the observed differences are considered. 3. Results obtained using Freeze-Etch electron microscopy and hemagglutination inhibition are given showing beef brain gangliosides in lipid vesicles to be effective receptors for influenza virus.