The amino-terminal fragment of gelsolin is cross-linked to Cys-374 of actin in the EGTA-resistant actin-gelsolin complex.

It has been shown that the EGTA-resistant actin, one of the two actin molecules associated to gelsolin, can be predominantly cross-linked to gelsolin by benzophenone-4-maleimide (BPM), a photoaffinity-labeling reagent, which was conjugated to Cys-374 of actin prior to cross-linking (Doi, Y., Banba, M. and Vertut-Doï, A. (1991) Biochemistry 30, 5769-5777). When a chymotryptic digest of gelsolin containing the amino-terminal 15-kDa fragment was mixed with BPM-actin (42 kDa) and irradiated for cross-linking, a band of 58 kDa appeared on SDS-PAGE which was shown to contain actin molecule by using fluorescently labeled actin. The amino-terminal sequence of the 58-kDa complex was identical to that of gelsolin, confirming that the amino-terminal segment (residues 1-133) of pig plasma gelsolin lies closely to Cys-374 of actin in the EGTA-resistant complex.