Phosphorylation and dephosphorylation of spectrin from human erythrocyte ghosts under physiological conditions: autocatalysis rather than reaction with separate kinase and phosphatase

A cyclic AMP-dependent phosphorylation of spectrin dimer

In contrast to the properties of spectrin obtained from [32P]phosphate-labeled red cells, purified spectrin dimer could be phosphorylated by a cAMP-dependent protein kinase from bovine heart. Both spectrin bands were phosphorylated. Spectrin band 2 contained in addition to autophosphorylated peptides several phosphopeptides that were distinct from autophosphorylated ones. The cAMP-dependent phosphorylation of spectrin band I was modulated by reducing agent and the concentration of spectrin. At high concentrations spectrin band 2 was predominantly labeled. The cAMP-dependent phosphoform of spectrin band 2 had a pI slightly higher than that of autophosphorylated spectrin band 2, but lower than that of ankyrin.

Reversible shape change of Triton-treated erythrocyte ghosts induced by Ca2+ and Mg-ATP

The shape of the human erythrocyte depends on intracellular ATP content and echinocytic erythrocyte ghosts obtained in the presence of Mg-ATP acquire a diskocytic shape after incubation at 37 degrees C; however, agreement is lacking about the molecular basis of the shape changes. The suggestion that phosphorylation of cytoskeletal structures underlying the membrane is involved has been disputed and alternative explanations based on lipid bilayer theory and metabolism of phospholipid have been proposed. Recently, we re-examined the effect of ATP on the shape of ghosts and found that it consists of two distinct steps. We have, therefore, now examined the effect of physiological concentrations of Ca2+ and ATP on the cytoskeleton of the erythrocyte membrane directly using Triton-treated ghosts which had lost this permeability barrier. Our findings suggest that a noncovalent effect of ATP on the cytoskeleton is a prerequisite for shape change.

Resensitization of lutropin-desensitized tumour Leydig-cell adenylate cyclase with human erythrocyte membranes

Purified rat tumour Leydig cells were pretreated with or without lutropin (1 h at 32 degrees C). The plasma membranes were then isolated and the adenylate cyclase activity measured in the presence of freshly prepared or heat-inactivated (1 h at 60 degrees C) human erythrocyte membranes. In plasma membranes from control cells in the presence of heat-inactivated human erythrocyte membranes both guanosine 5'-[beta, gamma-imido]triphosphate (p[NH]ppG) plus lutropin and NaF caused a 45--50-fold increase in cyclic AMP production over 30 min compared with 12--13 fold p[NH[ppG and 2--3-fold with lutropin alone. In plasma membranes isolated from lutropin-pretreated cells the NaF- and the p[NH]ppG-stimulated cyclic AMP production rates were unchanged, but no effect of lutropin could be demonstrated with or without added p[NH]ppG. However, after mixing lutropin-desensitized Leydig tumour-cell plasma membranes with freshly prepared human erythrocyte plasma membranes, the adenylate cyclase activity in the presence of lutropin, p[NH]ppG, lutropin plus p[NH]ppG and NaF were similar to those of control cell plasma membranes treated in the same manner. The possible mechanisms of this reversal of lutropin-induced desensitization by human erythrocytes are discussed.