Ciliary granule plaques: Membrane-intercalated particle aggregates associated with Ca2+-binding sites in Paramecium

Abstract

In Paramecium nine rectangular aggregates of membrane-intercalated particles surround the freeze-cleaved membrane of the ciliary base. These ‘ciliary granule plaques’ occur independently of the ‘ciliary necklace’ which is observed in a more basal position on some cilia. In each individual plaque the ∼ 10 nm large granules are arranged in a square grid pattern: the granules of one plaque invariably form 3 vertical rows (number of horizontally arranged particles per row: N_h = 3) and 3–6 horizontal rows (mean value for N_v = 4 ·8); the centre-to-centre spacing of granules is d_h = 22 nm in the horizontal and d_v = 24 in the vertical direction. The ciliary granule plaques coincide with electron-dense deposits which can be produced in a position adjacent to the inner side of the basal ciliary shaft membrane by 3 procedures: (a) when cells are fixed in glutaraldehyde after incubation with Ca²⁺ concentrations (up to 25 mM) which are higher than normally present in the medium (about 0·5 HIM) but which do not immobilize the animals prior to fixation in glutaraldehyde; (b) when Ca²⁺ (up to 75 mM) was added to glutaraldehyde; (c) when cells were incubated with Ca²⁺’ as in (a) and then fixed in OsO₄ solutions to which oxalate ions were added in concentrations equivalent to Ca²⁺. Comparable deposits were observed also with Ca²⁺ concentrations normally present in the medium, but they occurred much more rarely. The occurrence of electron-dense deposits was not enhanced by addition of Na⁺, Mg²⁺ or La³⁺ instead of Ca²⁺. In cross-sectioned ciliary bases the electron-dense Ca²⁺-dependent deposits face the 9 doublet microtubules. Occasionally the substructure of Ca²⁺-dependent deposits is resolved in median sections; their periodicity in the vertical direction (d_v = 25nm; N_v = 5 ·2) corresponds to the periodicity of membrane-intercalated particles of freeze-cleaved plaques. Other parameters determined for Ca²⁺-dependent electron-dense deposits coincide also with those determined for freeze-cleaved ciliary granule plaques (values in parentheses are for cells with Ca²⁺ added before fixation with glutaraldehyde and ultrathin sectioning): The height of plaques (or deposits) is 84(115) nm; the distance from their lower end to the ciliary base is 138 (121) nm. Plaques (or deposits) are 45 (44) nm broad and the distance separating adjacent plaques (deposits) is 48 (50) nm. Plaques (deposits) are arranged with a horizontal periodicity of mean 93–102 (92) nm. It is concluded that the ciliary granule plaques are identical to or associated with Ca²⁺-binding sites involved in ciliary activity or other functions.