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Where does Mn bind when it triggers the luminescence of Ca-regulated photoproteins? BA Illarionov2; VA Illarionova2; VS Bondar2; ES Vysotski2; John R Blinks*1 (1) Friday Harbor Labs., Univ. of Washington, Friday Harbor, WA 98250, USA; (2) Photobiology Lab., Institute of Biophysics, Krasnoyarsk 660036, Russia      *(blinks@fhl.washington.edu) Mn2+ is capable of stimulating photoprotein luminescence, though to lower maximum levels than Ca2+. All calcium-regulated photoproteins contain 3 binding sites of the EF-hand type. However, EPR results from both aequorin and obelin have been interpreted as indicating that only one Mn binds per molecule of photoprotein. If Mn binds to one of the EF-hand sites, and if Mn binding is interfered with by the same mutations that disable Ca binding, it should be straightforward to establish which of the three sites binds Mn. We determined concentration-effect curves for Mn on two sets of mutants of recombinant obelin in which the Ca-binding sites had been disabled in different ways (by removing the side chain carboxyl group in the 1st or the 12th position of the 12-amino-acid EF-loop). We obtained similar results with both sets of mutants: disabling site I (nearest the N-terminus) actually increased Mn-induced luminescence slightly; disabling site III greatly reduced the ability of Mn to stimulate luminescence, but did not eliminate it. However, disabling site II produced an intermediate result, suggesting that Mn can interact with site II as well as site III. It is not yet clear to us how this surprising finding can be reconciled with the EPR results. (Support: NIH, WA Sea Grant, Russ. Acad. Sci.) [Poster: blinks.johnr.62961]

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