Microtubule dissassembly in vivo: intercalary destabilization and breakdown of microtubules in the heliozoan Actinocoryne contractilis.

In the marine heliozoan Actinocoryne contractilis, uninterrupted rods of microtubules stiffen the axopodia and the stalk. Stimulation in sea water elicits an extremely fast contraction (millisecond range) accompanied by almost complete Mt dissociation. Using high-speed cinematography and light transmittance measurements, we have studied the process of Mt disassembly in real time. In sea water, Mt disassembly follows an exponential decrease (mean half time of 4 ms) or proceeds by short steps. Cell contraction and Mt disassembly have been inhibited or slowed down through the use of artificial media. Although kinetics are slower (mean half time of 3 s), the curves of the length change against time look similar. The rapid as well as the slower process are accompanied by the formation of breakpoints on the stalk, from which disassembly proceeds. In specimens fixed during the slowed contraction, the presence across the Mt rods, of a single or multiple destabilization band that may consist of granular material and polymorphic forms of tubulin supports the hypothesis of "intercalary destabilization and breakdown" of axonemal Mts.

The myoneme of the Acantharia (Protozoa): A new model of cellular motility.

The myonemes of Acantharia are made of bundles of microfilaments twisted up in elementary microstrands of two. Myonemes exhibit three kinds of movements: slow contraction accompanied by undulations of the organelle, quick contraction and subsequent relaxation. In vivo, contraction-relaxation cycles were studied with normal, time-lapse and high speed film-recordings (1000 frames/s). Kinetic parameters of these movements are given. All the movements are actively produced by the organelle itself and are dependant on calcium concentration. An hypothesis about the way myoneme microfilaments might interact to generate motile forces is given. The most closely related system might be the myoneme of the dinoflagellate Noctiluca miliaris.