IDENTIFICATION OF THE BINDING AND INHIBITION SITES IN THE CALMODULIN MOLECULE FOR OPHIOBOLIN-A BY SITE-DIRECTED MUTAGENESIS

Ophiobolin A, a fungal toxin that affects maize and rice, has previous ly been shown to inhibit calmodulin by reacting with the lysine (Lys) residues in the calmodulin. In the present study we mutated Lys-75, Ly s-77, and Lys-148 in the calmodulin molecule by site-directed mutagene sis, either by deleting them or by changing them to glutamine or argin ine. We found that each of these three Lys residues could bind one mol ecule of ophiobolin A. Normally, only Lys-75 and Lys-148 bind ophiobol in A. Lys-77 seemed to be blocked by the binding of ophiobolin A to Ly s-75. Lys-75 is the primary binding site and is responsible for all of the inhibition of ophiobolin A. When Lys-75 was removed, Lys-77 could then react with ophiobolin A to produce inhibition. Lys-148 was shown to be a binding site but not an inhibition site. The Lys-75 mutants w ere partially resistant to ophiobolin A. When both Lys 75 and Lys-77 o r all three Lys residues were mutated, the resulting calmodulins were very resistant to ophiobolin A. Furthermore, Lys residues added in pos itions 86 and/or 143 (which are highly conserved in plant calmodulins) did not react with ophiobolin A. None of the mutations seemed to affe ct the properties of calmodulin. These results show that ophiobolin A reacts quite specifically with calmodulin.