Localization of fluorescence-labeled poly(malic acid) to the nuclei of the plasmodium of Physarum polycephalum.

The nuclei in the plasmodium of Physarum polycephalum, as of other myxomycetes, contain high amounts of polymalate, which has been proposed to function as a scaffold for the carriage and storage of several DNA-binding proteins [Angerer, B. and Holler, E. (1995) Biochemistry 34, 14741-14751]. By delivering fluorescence-labeled polymalate into a growing plasmodium by injection, we observed microscopic staining of nuclei in agreement with the proposed function. The fluorescence intensity was highest during the reconstruction phase of the nuclei. To examine whether the delivery was under the control of polymalatase or related proteins [Karl, M. & Holler, E. (1998) Eur. J. Biochem.251, 405-412], the cellular distribution of these proteins was also examined by staining with antibodies against polymalatase. Double-stained plasmodia revealed a fluorescent halo around each fluorescent nucleus during the reconsititution. Fluorescent nuclei were not observed when the hydroxyl terminus of polymalate, known to be essential for the binding of polymalatase, was blocked by labeling with fluorescein-5-isothiocyanate. By immune precipitation, it was shown that polymalate and polymalatase or related proteins were in the precipitate. It is concluded that polymalate is delivered to the surface of nuclei in the complex with polymalatase or related proteins. The complex dissociates, and polymalate translocates into the nucleus, while polymalatase or related proteins remain at the surface.