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Changes in cell surface glycoproteins on non-differentiating L6 rat myoblasts selected for resistance to concanavalin
Abstract:
Four independently selected conA-resistant, non-differentiating rat L6 myoblast cell lines and their parental wild-type populations were examined for cell surface alterations. [3H]conA-binding studies indicated that the variant myoblasts bound significantly less lectin than wild-type cells at 4 and at 37 degrees C. Scatchard analysis revealed two general types of binding sites (high and low affinity sites) on wild-type cells; the variants appeared to be deficient in the high affinity sites. These changes in conA binding probably play an important role in determining the conA-resistant phenotype. Lectin-binding results could be significantly modified by altering the composition of the serum in the growth medium used to culture myoblasts prior to performing binding experiments, suggesting the existence of productive and non-productive lectin-binding sites on the cell surface. SDS slab gel electrophoresis of [3H]mannose-labelled surface membranes prepared from variant and wild-type cells showed that several glycoproteins of the conA-resistant myoblasts were defective in mannosylation. The conA-binding abilities of a pronase digest of one of these altered regions from variant separations, with a molecular weight of 44500 D, was found to contain glycopeptides with reduced affinity for the lectin, supporting the idea that variant membranes are deficient in a set of high affinity lectin-binding sites. Studies on [GDP-14C]mannose incorporation into lipid by membranes from variant and wild-type myoblasts indicated that the biosynthetic lesion likely involved a mannosyl transferase enzyme directly, rather than a lack of free dolichol-PO4. These studies link conA resistance, cell surface glycoprotein alterations, and defective mannosyl transferase activity with the inability to carry out normal cellular differentiation to form multinucleated myotubes.
Insights
Concanavalin A (ConA)-resistant rat myoblasts show altered cell surface glycoproteins and defective mannosyl transferase activity, hindering differentiation into multinucleated myotubes.
Area of Science:
- Cell Biology
- Biochemistry
- Glycobiology
Background:
- Rat L6 myoblast cell lines resistant to concanavalin A (ConA) and their wild-type counterparts were analyzed for cell surface changes.
- ConA resistance is often associated with alterations in cell surface glycoproteins and carbohydrate-binding proteins.
Purpose of the Study:
- To investigate the molecular basis of ConA resistance in rat L6 myoblasts.
- To identify specific cell surface alterations and underlying biochemical defects responsible for the resistant phenotype and impaired differentiation.
Main Methods:
- ConA binding assays at different temperatures (4°C and 37°C) and Scatchard analysis to characterize lectin-binding sites.
- SDS-PAGE of surface glycoproteins labeled with [3H]mannose.
- Analysis of [GDP-14C]mannose incorporation into lipids by membrane preparations.
Main Results:
- Variant myoblasts exhibited significantly reduced ConA binding compared to wild-type cells, primarily due to a deficiency in high-affinity binding sites.
- SDS-PAGE revealed defects in mannosylation of several surface glycoproteins in ConA-resistant variants.
- Studies indicated that the biosynthetic defect likely involves a mannosyl transferase enzyme, not a lack of dolichol phosphate.
Conclusions:
- ConA resistance in rat L6 myoblasts is linked to alterations in cell surface glycoproteins and defective mannosyl transferase activity.
- These molecular defects contribute to the inability of the variant cells to differentiate into multinucleated myotubes.