Chemical modification of contractile 3-nm-diameter filaments in Vorticella spasmoneme by diethyl-pyrocarbonate and its reversible renaturation by hydroxylamine

Abstract A peritrich ciliate possesses a zooid and a long stalk consisting of a bundle of 3-nm-diameter filaments. Glycerinated stalks can contract in the presence of free Ca 2+ and re-extend in the absence of free Ca 2+ . In the present study, we demonstrated that histidine residue(s) played a critical role in spasmoneme contraction by using glycerinated stalk of Vorticella . Concentration-dependent inhibition of spasmoneme contraction was observed in the presence of reversible histidine-modifying reagent named diethyl-pyrocarbonate (DEPC). In addition, the contractility degree of DEPC-modified spasmoneme could be partially restored by hydroxylamine treatment. The 244 nm absorption of modified spasmoneme protein(s) increased with rising DEPC concentration and decreased following the addition of hydroxylamine treatment. Adding Ca 2+ before DEPC modification could prevent the spasmoneme contraction from inhibition of DEPC. Those results suggested that histidine residues were actively involved in spasmoneme contraction. Ca 2+ -binding ability of spasmin was not inhibited by DEPC modification, which suggested that the essential histidine residues were not on the calcium-binding site of spasmin.