Modification of Cysteine Residues by N-Ethylmaleimide Inhibits Annexin II Tetramer Mediated Liposome Aggregation

A role of cysteine residues in annexin II tetramer (AIIt)ʹs function was investigated using the sulfhydryl reagent N-ethylmaleimide (NEM). Incubation of AIIt with NEM resulted in a dose-dependent inhibition of AIIt-mediated liposome aggregation and loss of sulfhydryl groups of AIIt. The concentration effecting 50% inhibition was 0.18 mM. The inhibition was observed in all Ca2+ concentrations tested (1–1000 μM). NEM had no effects on liposome aggregation mediated by other annexins (I, III, and IV), indicating that the inhibitory effect caused by NEM modification is specific to AIIt. The NEM-treated AIIt still can bind to liposomes. However, once AIIt was bound to membrane, the cysteine residues were protected from NEM modification. Our results suggest that cysteine residues are critical for AIIt-mediated liposome aggregation.