Left: ultra-structure of a kinetocyst of Raphidiophrys contractilis); right: two kinetocysts in an axopodium.
In centrohelid heliozoa, extrusomes are called kinetocysts, and are present in close association with the plasma membrane of axopodia.
A kinetocyst is an extrusive organelle that discharges its contents upon food capture. The posterior part of a discharged kinetocyst is always attached to the plasma membrane of the heliozoon, while the anterior end is directed towards the prey, often making close contact with a food organism. This association possibly provides a cell surface scaffold that aids in extension of pseudopodia during formation of the food cup.
A kinetocyst is a globular organelle that measures about 0.4 µm in height and 0.3 µm in width, and is surrounded by a membrane. It contains a bipartite central element (core) enclosed by a jacket of less electron dense material with fine striations, most likely being composed of stacked disks. The central core and the jacket are covered with a mushroom-shaped cap structure that is associated with the surrounding membrane at its tapered edges. Filamentous antennae are located outside the plasma membrane with apparent structural connection to the central core.
During prey capture, kinetocysts expel some material towards prey organisms. After discharge, the basal part of the kinetocyst keeps its association with the surrounding membrane, which, as a result of exocytosis of the kinetocyst, becomes incorporated into the plasma membrane. The jacket of discharged kinetocysts spread out and transform into a much less distinct structure, while the appearance of the central core remains unchanged. The distal end of the core is connected to or fused with the cell surface of the prey.
Reference:Sakaguchi, M., Suzaki, T., Kamal Khan, S.M.M. and Hausmann, K. (2002). Food capture by kinetocysts in the heliozoon Raphidiophrys contractilis. Europ. J. Protistol. 37, 453–458.
Ferry Siemensma, created October 30, 2019; last modified January 31, 2021