Primary structure of diphtheria toxin fragment B: structural similarities with lipid-binding domains.

Abstract

Two different lipid-associating domains were identified in the B fragment of diphtheria toxin using automated Edman degradation of its cyanogen bromide peptides, secondary structure prediction analysis and comparisons with known phospholipid-interacting proteins. The 1st domain is located in the highly hydrophilic (polarity index [PI] = 61.0%) 9000-dalton N-terminal region of fragment B. This region shows primary and predicted secondary structures dramatically similar to those found for the phospholipid headgroup-binding domains of human apolipoprotein A1 (surface lipid-associating domain). The 2nd domain is located in the highly hydrophobic (PI = 32.4%) middle region of fragment B. Its structure resembles that found for the membranous domain of intrinsic membrane proteins (transverse lipid-associating domain). The hydrophilic C-terminal 8000-dalton region of fragment B (PI = 53.8%) does not show structural similarity with lipid-associating domains.