INCREASED MEMBRANE-PROTEIN METHYLATION IN HEREDITARY SPHEROCYTOSIS - A MARKER OF CYTOSKELETAL DISARRAY

Protein carboxyl methyltransferase of type II selectively recognizes L -isoaspartyl and D-aspartyl residues spontaneously occurring in protei ns and peptide substrates. Membrane protein methylation levels increas e with erythrocyte aging in circulation, in parallel with the spontane ous formation of abnormal aspartyl sites, due to protein intrinsic ins tability. We found that enzymic methyl esterification of erythrocyte m embrane proteins in hereditary sphero-cytosis, a model of cytoskeletal disarray, is significantly increased compared to normal red blood cel ls. This cannot be explained by an increase in mean age of spherocytes , which are on the contrary significantly younger than control cells. No differences in cytosolic methyltransferase specific activity, as we ll as in the intracellular concentrations of the methyl donor adenosyl methionine and/or of the methylation inhibitor adenosylhomocysteine we re observed. We identified bands 2.1, 4.1 and 4.2 as the main targets for increased methylation, whose levels were correlated with the degre e of spectrin deficiency associated with this anemia. Our findings ind icate that membrane-protein methyl esterification represents a marker of membrane structural alteration in vivo in spherocytosis. We hypothe size that either an increased accessibility of methylation sites norma lly not available to the methyltransferase, or accelerated formation o f methyl-accepting sites in membrane proteins are present in spherocyt osis.