Interrelationships between protein kinases and spectrin phosphorylation in human erythrocytes

The use of erythrocytic and animal models in the study of protein phosphorylation

Phosphorylation processes are common post-transductional mechanisms, by which it is possible to modulate a number of metabolic pathways. Proteins are highly sensitive to phosphorylation, which governs many protein-protein interactions. The enzymatic activity of some protein tyrosine-kinases is under tyrosine-phosphorylation control, as well as several transmembrane anion-fluxes and cation exchanges. In addition, phosphorylation reactions are involved in intra and extra-cellular 'cross-talk' processes. Early studies adopted laboratory animals to study these little known phosphorylation processes. The main difficulty encountered with these animal techniques was obtaining sufficient kinase or phosphatase activity suitable for studying the enzymatic process. Large amounts of biological material from organs, such as the liver and spleen were necessary to conduct such work with protein kinases. Subsequent studies revealed the ubiquity and complexity of phosphorylation processes and techniques evolved from early rat studies to the adaptation of more rewarding in vitro models. These involved human erythrocytes, which are a convenient source both for the enzymes, we investigated and for their substrates. This preliminary work facilitated the development of more advanced phosphorylative models that are based on cell lines.

Modulation of erythrocyte membrane mechanical function by beta-spectrin phosphorylation and dephosphorylation

The mechanical properties of human erythrocyte membrane are largely regulated by submembranous protein skeleton whose principal components are alpha- and beta-spectrin, actin, protein 4.1, adducin, and dematin. All of these proteins, except for actin, are phosphorylated by various kinases present in the erythrocyte. In vitro studies with purified skeletal proteins and various kinases has shown that while phosphorylation of these proteins can modify some of the binary and ternary protein interactions, it has no effect on certain other interactions between these proteins. Most importantly, at present there is no direct evidence that phosphorylation of skeletal protein(s) alters the function of the intact membrane. To explore this critical issue, we have developed experimental strategies to determine the functional consequences of phosphorylation of beta-spectrin on mechanical properties of intact erythrocyte membrane. We have been able to document that membrane mechanical stability is exquisitely regulated by phosphorylation of beta-spectrin by membrane-bound casein kinase I. Increased phosphorylation of beta-spectrin decreases membrane mechanical stability while decreased phosphorylation increases membrane mechanical stability. Our data for the first time demonstrate that phosphorylation of a skeletal protein in situ can modulate physiological function of native erythrocyte membrane.

Maturation-related change of the protein kinase activity in the rabbit red blood cells

The protein kinase activity of rabbit reticulocyte and erythrocyte hemolysate is due to multiple forms (casein kinases and histone kinases) which have been partially purified by Sepharose 6B filtration followed by phosphocellulose chromatography in the presence of 0.2% Triton X-100. The casein kinase activity is resolved by such a procedure into two forms differing in catalytic properties: i.e. the casein kinase TS phosphorylates both serine and threonine residues of casein, while the casein kinase S phosphorylates only serine residues. The comparative results indicate that during differentiation and maturation of rabbit red blood cells multiple cytosol protein kinase activities markedly decrease, while the casein kinase S does not change significantly.

In vitro phosphorylation of the red blood cell cytoskeleton complex by cyclic AMP-dependent protein kinase from erythrocyte membrane

Hydrosoluble proteins extracted from human erythrocyte ghosts by dialysis at low ionic strength and alkaline pH contain a cyclic AMP-dependent protein kinase which phosphorylates in vitro the cytoskeleton components in crude extracts. Spectrin components 1 and 2, actin and protein band 4.1 undergo this cyclic AMP-dependent endogenous phosphorylation together with low molecular weight peptides solubilized with the cytoskeleton in hydrosoluble extract. However, pure spectrin and purified erythrocyte G-actin were not phosphorylated by purified cyclic AMP-dependent protein kinase from erythrocyte membrane. Purified G-actin when added free to crude extract does not undergo phosphorylation by the cyclic AMP-dependent protein kinase present in this extract. In contrast, purified cyclic AMP-dependent protein kinase added either to crude extract or to the purified cytoskeleton complex (spectrin, actin and protein band 4.1), phosphorylates spectrin, actin and protein band 4.1. We can conclude that (1) cyclic AMP-dependent phosphorylation of red cell cytoskeleton occurs in vitro only when the cytoskeleton components are in a complexed form; (2) red cell actin, like other cellular actins, may be phosphorylated by cyclic AMP-dependent protein kinase but only in the oligomeric form and not in the G form.

Further purification and characterization of casein kinases from human erythrocyte hemolysate. Effect of Triton X-100

Two cyclic AMP-independent casein kinases can be isolated from human erythrocyte hemolysate, one of which (referred to as 'casein kinase S') phosphorylates only serine residues of whole commercial casein, while the other (referred to as 'casein kinase TS') phosphorylates both serine and threonine residues of the same substrate. Moreover, the casein kinase S, unlike casein kinase TS, is able to phosphorylate the erythrocyte membrane proteins. The present paper deals with the further characterization of casein kinase S, freed from histone kinase activity by DEAE and subsequent phosphocellulose chromatography of the crude hemolysate in the presence of 0.2% Triton X-100. In particular, cytosol casein kinase S exhibits some physico-chemical and catalytic properties identical to those of the membrane-bound casein kinase, solubilised and purified as previously described. Both casein kinases display the same chromatographic behaviour, the same Sepharose elution volume, the same optimal pH range, the same Km for casein and ATP, the same response to NaCl, MgCl2 and CaCl2, and the same ability to phosphorylate serine but not threonine residues of beta-casein.