Jemal C, Haddad J E, Begum D, Jackson M P
Department of Immunology and Microbiology, Wayne State University School of Medicine, Detroit, Michigan 48201, USA.
J Bacteriol. 1995 Jun;177(11):3128-32. doi: 10.1128/jb.177.11.3128-3132.1995.
Shiga toxin (STX), a bacterial toxin produced by Shigella dysenteriae type 1, is a hexamer composed of five receptor-binding B subunits which encircle an alpha-helix at the carboxyl terminus of the enzymatic A polypeptide. Hybrid toxins constructed by fusing the A polypeptide sequences of STX and Shiga-like toxin type II were used to confirm that the carboxyl terminus of the A subunits governs association with the B pentamers. The alpha-helix of the 293-amino-acid STX A subunit contains nine residues (serine 279 to methionine 287) which penetrate the nonpolar pore of the B-subunit pentamer. Site-directed mutagenesis was used to establish the involvement of two residues bordering this alpha-helix, aspartic acid 278 and arginine 288, in coupling the C terminus of StxA to the B pentamer. Amino acid substitutions at StxB residues arginine 33 and tryptophan 34, which are on the membrane-contacting surface of the pentamer, reduced cytotoxicity without affecting holotoxin formation. Although these B-subunit mutations did not involve receptor-binding residues, they may have induced an electrostatic repulsion between the holotoxin and the mammalian cell membrane or disrupted cytoplasmic translocation.
志贺毒素（STX）是由1型痢疾志贺菌产生的一种细菌毒素，是一种六聚体，由五个受体结合B亚基组成，这些亚基围绕着酶促A多肽羧基末端的α螺旋。通过融合STX和II型志贺样毒素的A多肽序列构建的杂合毒素被用于证实A亚基的羧基末端控制着与B五聚体的结合。293个氨基酸的STX A亚基的α螺旋包含九个残基（丝氨酸279至甲硫氨酸287），它们穿透B亚基五聚体的非极性孔。定点诱变被用于确定与该α螺旋相邻的两个残基，天冬氨酸278和精氨酸288，在将StxA的C末端与B五聚体偶联中的作用。位于五聚体膜接触表面的StxB残基精氨酸33和色氨酸34处的氨基酸取代降低了细胞毒性，而不影响全毒素的形成。尽管这些B亚基突变不涉及受体结合残基，但它们可能在全毒素和哺乳动物细胞膜之间诱导了静电排斥或破坏了细胞质转运。
