Spectral analysis of fluorescence of 8-anilino-1- naphthalenesulfonate in chemoreception with a white plasmodium of Physarum polycephalum Evidence for conformational change in chemoreceptive membrane

Abstract

A plasmodial mutant (strain Lu 647 × Lu 861) of Physarum polycephalum contains much less yellow pigment compared with the wild type. Thus, extrinsic fluorescence measurements should be possible with the use of this white plasmodium. We adopted 8-anilinonaphthalenesulfonate (ANS) as a fluorescent anion, and measured fluorescence spectra and polarization in conjunction with the membrane potential when the cell was stimulated with various salts. Results are: 1. (1) The membrane potential, Δϕ, did not change until the concentration of stimulus chemicals exceeded respective thresholds, C th . Above C th , Δϕ depolarized linearly with log C. The Schulze-Hardy rule was applicable to C th ; i.e. log C th = − log z + log K H where z is the valency of the cations. The Hamaker constant, K H , displayed two distinct values depending on the cation species: K H = 3 mM for Ca, La and Th (Group I) and K H = 0.2 mM for Na, K, NH 4, Mg, Ba and Al (Group II). The value of K H in the wild type was 3 mM for all polyvalent cations examined. 2. (2) When excited at 360 nm, ANS gave two emission maxima of 500–510 nm and 460 nm through interaction with the plasmodial membrane. Application of an Mg or Na salt of ANS below the respective C th gave a single peak at 500 nm, while that above C th caused an increase in fluorescence intensity at 460 nm. 3. (3) Fluorescence polarization, p, at 460 nm decreased appreciably when an Mg or Na salt of ANS above the respective C th was applied to the plasmodium. Addition of CACl 2, LaCl 3 or Th(NO 3) 4 caused an increase in the p value, while that of KCl, NaCl, MgCl 2 or AlCl 3 did not. Results described above indicate that the plasmodial membrane undergoes a conformational change upon salt reception. Based on the physicochemical properties of ANS, we may conclude that chemoreceptive membrane of the plasmodium becomes more hydrophobic above C th .