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Surface aspartate residues are essential for the stability of colicin A P-domain: A mechanism for the formation of an acidic molten-globule

Lookup NU author(s): Susan Fridd, Professor Jeremy LakeyORCiD


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The pore-forming domains of members of a family of bacterial toxins, colicins N and A, share > 50% sequence identity, identical folds and yet display strikingly different behavior in acid conditions. At low pH colicin A forms a molten globule state while colicin N retains a native fold. This is relevant to in vivo activity since colicin A requires acidic phospholipids for its toxic activity but colicin N does not. The pI of colicin A (5.25) is far lower than that of colicin N (10.2) because colicin A contains seven extra aspartate residues. We first introduced separately each of these acidic amino acids into homologous sites in colicin N, but none caused destabilization at low pH. However, in the reverse experiment, the sequential replacement of these acidic side chains of colicin A by alanine revealed six sites where this change destabilized the protein at neutral pH. Some of these residues, which each contribute less than 4% to the total negative charge, appear to stabilize the protein via a network of hydrogen bonds and charge pairs which are sensitive to protonation. Other residues have no clear interactions that explain their importance. The colicin A is thus a protein that relies upon acid sensitive interactions for its stability at neutral pH and its in vivo activity.

Publication metadata

Author(s): Fridd SL, Lakey JH

Publication type: Article

Publication status: Published

Journal: Biochemistry

Year: 2002

Volume: 41

Issue: 5

Pages: 1579-1586

ISSN (print): 0006-2960

ISSN (electronic): 1943-295X

Publisher: American Chemical Society


DOI: 10.1021/bi015633k

PubMed id: 11814351


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