STUDY OF AL3-SUBSTITUTED C-TERMINAL DOMAIN OF THE NF-M PROTEIN AND ITS RELEVANCE TO ALZHEIMERS-DISEASE( BINDING AND CONFORMATIONAL PROPERTIES OF THE ALANINE)

NF-M13 -Pro-Val-Pro-Lys-Ser-Pro-Val-Glu-Glu-Lys-Gly)-OH], NF-M17 -Pro- Val-Pro-Lys-Ser-Pro-Val-Glu-Glu-Lys-Gly)-OH], and their phosphorylated derivatives, representing the C-terminal phosphorylation domain of th e neurofilament protein midsize subunit, have four possible binding si tes for metal ions: the COO- group of glutamate, the OH group of the s erine residue, the PO3H- group of phosphoserine (when present), and th e COO- at the terminus of the peptide chain. The CD titration of the p hosphorylated neurofilament fragments with Al3+ and Ca2+ yielded a sig nificant conformational change that resulted in conformations containi ng high beta-pleated-sheet contents, which precipitate on standing (in termolecular complex). Al3+ binding to the unphosphorylated NF-M13 and NF-M17 did not exhibit this behavior. Several alanine analogues of th e parent NF-M17 peptide were synthesized in order to determine the rel ationship between metal ions and possible binding sites. CD titration of analogues with Ca2+ indicated that the critical residues of NF-M17 for Ca2+-induced conformational changes, from random to beta-pleated s heet, are the N-terminal serine or both phosphorylated serines. Al3+-i nduced conformational changes suggest that the critical sites of NF-M1 7 yielding the beta-pleated-sheet structure are the four glutamates or phosphorylated serines, especially the C-terminal SerP. On the basis of the titration data, it is very likely that analogues with a serine in position 11 form a stable intramolecular complex with Al3+ that, ho wever, does not result in the adoption of the beta-conformation. Back- titration with citric acid fails to reverse the Al3+-induced conformat ional changes of the phosphorylated peptides. The above results, espec ially the possible formation of intramolecular and intermolecular Al3 complexes, may have relevance to the molecular mechanism, through whi ch the neurotoxin Al3+ gives rise to the formation of neurofilament ta ngles.