Contribution of cysteine 158, the glycosylation site theonine 194, the amino- and carboxy-terminal domains of apolipoprotein E in the binding to amyloid peptide beta (1-40)

Recent studies have shown that at physiological conditions (pH 7.6, 37 degr ees C), the reactivity of recombinant apoE isoforms secreted by mammalian c ells toward amyloid peptide beta (AP40) follows the order apoE2 > apoE3 > a poE4 for the apoE monomer and apoE2 > apoE3 for apoE dimer that is formed v ia that intramolecular disulfide bridges. Different A beta binding properti es have been reported for the plasma-derived apoE and commercially availabl e apoE preparations that differ from the native apoE forms in the degree of their O-glycosylation. To define structural:elements of apoE involved in t he interaction with A beta, we have introduced point mutations as well as a mino- and carboxy-terminal deletions in the apoE structure. The mutant apoE forms were expressed transiently using the Semliki Forest Virus system,and the culture medium was utilized to study the reactivity of the mutated pro teins with A beta 40. This analysis showed that a mutation in the O-glycosy lation site of apoE2 (Thr194-Ala) did not affect the SDS-stable binding of apoE to A beta. In contrast, introduction of cysteine at:position 158 of ap oE4 (Arg112, Cys158) increased the SDS-stable binding of apoE to A beta to the levels similar to those observed in apoE2. Similar analysis showed that apoE truncated at residues 259, 249, 239, and 229 retains the SDS-stable b inding to A beta 40, whereas apoE truncated at residues 185 and 165 does no t bind to A beta. The deletion of aminoterminal residues 2-19 reduced the S DS-stable binding of apoE2 to A beta and deletion of residues 2-81 abolishe d binding to A beta. It is also noteworthy that the (Delta 2-81) apoE mutan t exists predominantly as a dimer, suggesting that removal of residues 2-81 promoted dimerization of apoE. These findings suggest that the amino- and carboxy-terminal residues of apoE are required for SDS-stable binding of ap oE to A beta and that the presence of at least one cysteine contributes to the efficient A beta binding.