title: Modification of actin with fluorescein isothiocyanate.
abstract: Reaction of rabbit skeletal muscle G-actin at pH 8.5 with fluorescein isothiocyanate (FITC) resulted in incorporation of up to 1.20 mol FITC/mol actin. At pH 8.8, the level of incorporation was raised to 1.98 mol FITC/mol actin. When excited with ultraviolet light, the FITC-actin samples fluoresced strongly with an emission maximum near 517 nm. Tryptic digests of FITC-actin containing about 1.0 mol FITC/mol actin could be separated into a nonfluorescent 33.5 kDa trypsin-resistant core protein and a fluorescent pool of small peptides. Chromatography on DEAE-Bio-Gel or two-dimensional separation on cellulose TLC plates of the peptide pool revealed that FITC was highly selective in the site of its reaction with actin, resulting in a single highly fluorescent peptide after tryptic digestion. NH2-terminal and amino acid analyses demonstrated this peptide to be derived from residues 51 to 62, with Lys-61 proposed as the major FITC-sensitive site on actin. FITC-actin is similar to G-actin in gross conformation; circular dichroism spectra of actin before and after labelling are identical. FITC-actin is also able to interact strongly with deoxyribonuclease I. However, FITC-actin solution viscosities and fluorescence properties are not altered by the addition of KCl or MgCl2. Therefore, either a localized conformational change near Lys-61 or steric hindrance due to the FITC attached to Lys-61 blocks the polymerization of actin.
mesh_terms: D000199:Actins / Q000378:metabolism*; D000596:Amino Acids / Q000032:analysis; D000818:Animals; D002942:Circular Dichroism; D003850:Deoxyribonuclease I / Q000378:metabolism; D016650:Fluorescein-5-isothiocyanate; D005452:Fluoresceins / Q000378:metabolism*; D011487:Protein Conformation; D011817:Rabbits; D013861:Thiocyanates / Q000378:metabolism*
chemical_list: D000199:Actins; D000596:Amino Acids; D005452:Fluoresceins; D013861:Thiocyanates; D003850:Deoxyribonuclease I; D016650:Fluorescein-5-isothiocyanate