Methylation of 21-23 kD membrane proteins by a membrane-associated protein carboxyl methyltransferase in neuroblastoma cells. Increased methylation in differentiated cells.
Membranes of neuroblastoma N1E-115 cells contain a specific protein carboxyl methyltransferase that methylates a 70 kD protein and a group of 21-23 kD proteins which are tightly bound to the membranes. The enzyme catalyzes the transfer of [methyl-3H] groups from [methyl-3H]S-adenosyl-L-methionine (Km = 0.22 microM) to these proteins to form base-labile carboxymethylesters. These protein methylesters are relatively stable compared to other protein methylesters, as shown by the ability of the 21-23 kD methylated proteins to retain their [methyl-3H] groups at pH values of 7 to 8.5 for at least 12 hr at room temperature. The extent of methylation of the 21-23 kD proteins, but not that of the 70 kD protein, was increased in membranes of cells induced to differentiate by 2% dimethyl sulfoxide (from a basal level of 0.1-0.2 to 0.9-1.2 pmol [methyl-3H] groups incorporated per mg membrane protein). This increase appeared after a lag period of 3 days of growth in the presence of the dimethyl sulfoxide and developed in parallel with the appearance of neurite-like processes in the cells. Kinetic experiments suggest that the amounts of 21-23 kD proteins available for methylation in the membranes of the undifferentiated and of the differentiated cells are limited. This and the previously observed low turnover of methylated 21-23 kD proteins in the intact cells suggest that the differentiated cells express and methylate more 21-23 kD proteins than the undifferentiated cells. These methylated proteins may be involved in differentiation or other functions of the differentiated cell membranes.
Department of Biochemistry, George S. Wise Faculty of Life Sciences, Tel Aviv University, Israel.
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R
Haklai
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Kloog
D000818:Animals; D002462:Cell Membrane / Q000201:enzymology / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism*; D008745:Methylation; D008780:Methyltransferases / Q000378:metabolism*; D051379:Mice; D009447:Neuroblastoma / Q000201:enzymology / Q000378:metabolism*; D014407:Tumor Cells, Cultured
D008565:Membrane Proteins; D008780:Methyltransferases