Kinetocysts

In centrohelid heliozoa, extrusomes are known as kinetocysts and occur in close association with the plasma membrane of the axopodia. A kinetocyst is an extrusive organelle that discharges its contents during prey capture. After discharge, the posterior part of the kinetocyst remains attached to the plasma membrane of the heliozoon, while the anterior end is directed toward the prey, often making close contact with the food organism. This association may provide a surface scaffold that aids in pseudopodial extension during formation of the food cup.

A kinetocyst is a globular organelle measuring approximately 0.4 µm in height and 0.3 µm in width, 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 composed of stacked disks. The central core and its jacket are capped by a mushroom‑shaped structure that connects to the surrounding membrane at its tapered edges. Filamentous antennae extend beyond the plasma membrane and appear structurally connected to the central core.

During prey capture, kinetocysts expel material toward the prey organism. After discharge, the basal part of the kinetocyst remains associated with the membrane, which becomes incorporated into the plasma membrane through exocytosis. The jacket of discharged kinetocysts spreads out and transforms into a much less distinct structure, while the central core remains largely unchanged. The distal end of the core becomes connected to, or fused with, the prey’s cell surface.

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.