Fat cell plasma membranes. II. Studies on the glycoprotein components

The major integral membrane glycoprotein in adipocytes is a novel 200-kDa heterodimer

The major glycoprotein in adipocytes was purified from rat adipocyte membranes by affinity chromatography with wheat germ agglutinin-agarose followed by DEAE-Sepharose ion exchange chromatography. The protein had an apparent molecular weight of 200-kDa when analysed by SDS-PAGE under non-reducing conditions. When electroeluted from the gel, boiled in the presence of beta-mercaptoethanol, and re-analysed by gel electrophoresis, it was found to be composed of 100- and 160-kDa subunits. The N-terminal sequences were determined through 20 amino acids for each subunit, were found to be identical, and were homologous with no previously described protein sequences. The protein was not extractable from the membrane by high salt concentrations, indicating it was an integral membrane protein. Membrane fractionation by differential ultracentrifugation showed it was present predominantly in the plasma membrane fraction. The protein was susceptible to cell surface radiolabelling, further suggesting it was a plasma membrane protein. In summary, the major membrane glycoprotein in adipocytes is a novel 200-kDa heterodimer whose disulfide-linked subunits possess identical N-terminal sequences.

Antibodies to ovine adipocyte plasma membranes recognize tissue and species specific plasma membrane components

1. Ovine adipocyte plasma membrane (PM) contains three unique proteins that have relative molecular mass of 70, 106, and 110 kD which are lacking in PM from liver, kidney, heart, and red blood cells. 2. Two major proteins on ovine adipocyte PM having molecular mass of 44 and 46 kD which were also present on porcine adipocyte PM. 3. These ovine proteins could not be detected on either rat or chicken adipocyte PM.

A comparison of the polypeptide composition of plasma membranes prepared from the white adipose tissue and adipocytes of the mouse, rat, rabbit, ox and chicken by a Percoll self-forming gradient procedure

The Percoll self-forming gradient method has been applied for the preparation of plasma membranes from white adipose tissue and isolated adipocytes from mouse, rat, rabbit, ox and chicken. Characterization of the membranes produced by marker enzyme analysis confirmed the general applicability of the method. The polypeptide composition of the membranes was studied by polyacrylamide gel electrophoresis. The patterns seen with tissue and adipocytes of a single species source were compared and the Coomassie blue and periodic acid Schiff staining bands characteristic of each species were compared systematically. Additionally materials from lean (FA/-) and obese (fa/fa) Zucker rats were compared. The species-specific differences in polypeptide composition form a useful basis for the further development of methods whereby the adipocyte-differentiated phenotype may be detected in a wide range of systems.

Concanavalin A receptors associated with rat brain synaptic junctions are high mannose-type oligosaccharides

Glycoproteins were isolated from a rat brain synaptic junction fraction by affinity chromatography on Concanavalin A-agarose. The isolated glycoproteins were digested with pronase and radiolabeled with 125I-Bolton Hunter reagent, and 125I-Concanavalin A-binding glycopeptides were isolated by chromatography on Concanavalin A-agarose. Treatment of the 125I-Concanavalin A-binding glycopeptides with either alpha-mannosidase or endo-beta-N-acetylglucosaminidase-C11 abolished their interaction with Concanavalin A. The pronase digest was reacted with endo-beta-N-acetylglucosaminidase-C11 and released oligosaccharides were reduced with NaB3H4. Following affinity chromatography on Concanavalin A-agarose, Concanavalin A-binding [3H]oligosaccharides were chromatographed on Biogel P4. Two major oligosaccharides corresponding to standard carbohydrates containing eight and five mannose residues were identified. Treatment of these oligosaccharides with alpha-mannosidase converted them to smaller saccharides having a mobility on Biogel P4 columns equal to the standard disaccharide mannose-beta-1-4-N'-acetylglucosamine. These results demonstrate that the Concanavalin A receptor activity associated with CNS synaptic junctions resides in asparagine-linked oligosaccharides of the high-mannose type.

Growth-dependent alterations in oligomannosyl glycopeptides expressed in Sindbis virus glycoproteins

Sindbis virus was used to examine host cell growth-dependent expression of oligomannosyl glycopeptides within single species of viral glycoproteins. Four mannosyl oligosaccharides ranging from 5 to 8 mannose residues were separated on Bio-Gel P4 column chromatography. Virus derived from rapidly growing chicken embryo fibroblasts displayed greater quantities of larger sized oligomannosyl glycopeptides in intact virus and in purified Sindbis virus glycoproteins E1 and E2 compared with virus from nongrowing cells. The pattern of mannosyl oligosaccharides in the two glycoproteins differed remarkably. E1 and E2 contained predominantly Man5GlcNAc and Man7GlcNAc, respectively; in addition, E1 relative to E2 contained more complex-type glycopeptides than mannosyl-type glycopeptides. These data clearly demonstrate that host-dependent alterations in glycosylation are expressed in Sindbis virus and that differences in specific glycosylation patterns are obscured in oligosaccharides derived from mixtures of glycoproteins. It is apparent that processing of these cotranslated glycoproteins can yield different patterns of oligosaccharide structures.