Synchronised Secretory Organelle Docking in Paramecium : Saltatory movement along microtubules transiently formed from ciliary basal bodies and selective exclusion of microinjected heterologous organelles

Abstract

In certain strains (nd) of Paramecium tetraurelia all secretory organelles (trichocysts) can be detached from the cell surface and then reattached synchronously. To account for the lability of microtubules involved in trichocyst docking during the preparation procedures we analysed this process by combining video microscopy, analogue contrast enhancement microscopy, laser scanning fluorescence microscopy (both after fast freezing, freeze-substitution and anti-tubulin antibody fluorescence labelling), and electron microscopy after cryofixation. We found that synchronous trichocyst docking is saltatory, occurring along transiently formed microtubules (not normally recognised in these cells), which emanate from ciliary basal bodies, acting as organising centres. Hence, trichocyst transport proceeds from the plus to the minus end of microtubules (in contrast to gland cells), with the ‘correct’ polarity (trichocyst tips pointing to the cell surface). We then injected chromaffin granules (isolated from bovine adrenal medullae) during the trichocyst detachment phase and analysed cells by electron microscopy during and after synchronised redocking of trichocysts. We used a chromate reaction for chromaffin granule identification on semithin sections by X-ray microanalysis (scanning transmission EM). While chromaffin granules remained intact (as judged by morphology and Cr signals) and although cell function was unimpaired (as judged by complete trichocyst attachment), we determined that heterologous organelle transport was not detectable, probably because of inverse microtubule polarity.