Chemical modification of charged amino acid moieties alters the electrophoretic mobilities of neurofilament subunits on SDS/polyacrylamide gels

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Abstract
The increase in the mobilities of neurofilament subunits on SDS-PAGE after dephosphorylation was reversed upon boiling in urea or trifluoroacetylation of lysine .epsilon.-amino groups. Trifluoroacetylation of native and dephosphorylated neurofilaments also resulted in an overall increase in the phosphorylation of the three subunits by the catalytic subunit of cyclic-AMP-dependent protein kinase. The gel-electrophoretic mobility of neurofilament subunits was also shown to be influenced by carboxylic amino acid residues, as neutralization of these moieties by glycinamidation increased the mobilities of all three subunits on SDS-PAGE. Neurofilament subunits that were both glycinamidated and dephosphorylated had apparent molecular masses of approximately 60 kDa, 112kDa and 138 kDa. The major sites of these changes in the two subunits were shown to be the carboxyterminal tail domains, which are known to contain high percentages of glutamate. Since interspecies differences in the apparent molecular masses of neurofilament subunits were shown to persist after glycinamidation and dephosphorylation, they appear to be due to differences in polypeptide chain length, rather than glutamate content.