Abstract
A thrombin-specific inhibitor with an apparent molecular mass of 11 kDa has been purified from the insect Rhodnius prolixus. Amino-terminal protein sequence analysis allowed the molecular cloning of the corresponding cDNA. The open reading frame codes for a protein of about 103 amino acid residues and displays an internal sequence homology of residues 6-48 with residues 57-101 indicating a two-domain structure. Based on the amino acid sequence the two domains exhibit high homology to protease inhibitors belonging to the Kazal-type family. Model building suggests that the first domain binds to the active site with residue His10 pointing into the specificity pocket. From gel filtration and tight-binding inhibition experiments the inhibitor appears to form 1:1 complexes with thrombin. Periplasma-directed heterologous expression of the rhodniin cDNA in Escherichia coli yields the intact thrombin inhibitor. Natural and recombinant rhodniin both display inhibition constants of about 2 x 10(-13) M.
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