Adenylate cyclase in a fibroblast mutant defective in glycolipid and glycoprotein synthesis

Abstract

THERE is currently wide interest in the biological role of glycolipids and glycoproteins of the cell membrane. As some transformed cells have reduced levels of the more complex gangliosides and neutral glycolipids (see refs 1–3 for reviews), it has been suggested that complex gangliosides and particularly ganglioside G_M1 may have an important role in regulating cell growth and in the responsiveness of adenylate cyclase (ATP pyrophosphate-lyase (cyclising), EC 4.6.1.1) to regulatory factors^3,4. The ganglioside G_M1 is thought to be required for the activation of plasma membrane adenylate cyclase by cholera toxin; the toxin activates adenylate cyclase⁵ and binds specifically to ganglioside G_M1 (refs 6–8). To investigate the effect of altered glycolipid and glycoprotein composition on adenylate cyclase activity we have used cell line AD6, a mutant of BALB/c 3T3 cells that is deficient in glucosamine phosphate acetyl transferase^9–12, and therefore partially blocked in its ability to synthesise complex carbohydrates, glycoproteins and glycolipids. Our results show that AD6 cells have a decreased responsiveness to cholera toxin, indicative of an altered membrane ganglioside composition, yet the responses of the enzyme to GTP, adrenaline, prostaglandin E₁ and fluoride are unaltered. Our results indicate that decreased synthesis of complex gangliosides does not result in alterations in adenylate cyclase other than a decreased response to cholera toxin.