Fig. 1. The C-terminal region of SWAP-70 binds to non-muscle actin in an isospecific manner. (A) The structure of SWAP-70 and its truncation mutants. The amino acid sequence of the putative actin-binding region in SWAP-70 was aligned with those of the homologous region seen in proteins of the gelsolin family. Basic amino acids are shown with a gray background; PH, pleckstrin-homology domain. (B) Binding of the C-terminal region of SWAP-70 to F-actin. His–SWAP-70(448-585) or His–SWAP-70(448-564) was mixed with or without (–) 10 µM polymerized actin derived from human platelets (non-muscle) or rabbit skeletal muscle (muscle), and ultracentrifuged. Proteins in the supernatant (S) and the pellet (P) were analyzed by SDS-PAGE followed by staining with Coomassie Blue. (C) Quantitative analysis for binding of the C-terminal region of SWAP-70 to F-actin. A co-sedimentation assay was performed by mixing of 5 µM polymerized non-muscle actin with various amounts of His–SWAP-70(448-585) at the indicated final concentrations. (D) Similar experiments as in C with a wide range of concentrations of His–SWAP-70(448-585) were performed, and the amounts of protein on the gel were quantified by an imaging analyzer.