Expression of bovine intestinal calcium binding protein from a synthetic gene in E. coli and characterization of the product

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Abstract
Intestinal calcium binding proteins (ICaBP''s) constitute a group of small vitamin D inducible proteins considered to play an important role in the absorption of dietary calcium. The mammalian ICaBP''s are representatives of the "EF-hand" family of calcium binding proteins. As a first step in the application of protein engineering techniques to the study of structure-function relationships in mammalian ICaBP''s, we have synthesized a gene encoding the minor A form (the native form lacking the two N-terminal amino acids) of bovine ICaBP employing a rapid, microscale gene synthesis technique based on "shotgun ligation" of sets of oligonucleotides. Expression of the synthetic gene from a plasmid containing the tac promoter in a lon protease deficient strain of Escherichia coli yielded the desired product at a level of about 1-2 wt % of total protein. During the purification of the ICaBP expressed in E. coli, a contaminant was strongly adhering to it but was efficiently removed by gel filtration after denaturation with urea. The minor A form of ICaBP produced in E. coli was characterized by its mobility during sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by its total amino acid composition, partial amino acid sequence, UV absorption spectrum, and 360-MHz 1H NMR spectrum, showing beyond reasonable doubt its identity with the minor A form of ICaBP obtained from bovine intestines.