Protein Science, Vol 6, Issue 5 1110-1113, Copyright © 1997 by Cold Spring Harbor Laboratory Press

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Identification and structural influence of a differentially modified N-terminal methionine in human S100b

S. P. SMITH, K. R. BARBER and G. S. SHAW
Department of Biochemistry and McLaughlin Macromolecular Structure Facility, The University of Western Ontario, London, Ontario N6A 5C1, Canada

The calcium-binding protein S100b is a homodimer comprised of two identical 91-residue {beta}-subunits. Recombinant S100b is a heterogeneous protein, although the basis of this heterogeneity has not been established. We have used mass spectrometry and NMR spectroscopy to determine that heterogeneity in S100b arises from a mixture of formyl-S100b and desformyl-S100b when expressed in Escherichia coli. Reversed-phase HPLC purification of these two forms of S100b has allowed the differences in N-terminal composition to be used as a probe for tertiary contacts in the protein. The presence or absence of the N-terminal formyl group affected the chemical shifts of sequence neighboring residues and those in the linker of the protein (residues 40-43), indicating that these two regions are close in space.
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