IN-VITRO MODELS OF TAIL CONTRACTION AND CYTOPLASMIC STREAMING IN AMEBOID CELLS

We have developed a reconstituted gel-sol and contractile model system that mimics the structure and dynamics found at the ectoplasm/endopla sm interface in the tails of many amoeboid cells. We tested the role o f gel-sol transformations of the actin-based cytoskeleton in the regul ation of contraction and in the generation of endoplasm from ectoplasm . In a model system with fully phosphorylated myosin II, we demonstrat ed that either decreasing the actin filament length distribution or de creasing the extent of actin filament cross-linking initiated both a w eakening of the gel strength and contraction. However, streaming of th e solated gel components occurred only under conditions where the leng th distribution of actin was decreased, causing a self-destruct proces s of continued solation and contraction of the gel. These results offe r significant support that gel strength plays an important role in the regulation of actin/myosin II-based contractions of the tail cortex i n many amoeboid cells as defined by the solation-contraction coupling hypothesis (Taylor, D. L., and M. Fechheimer. 1982. Phil. Trans. Soc. Lond. B. 299:185-197). The competing processes of solation and contrac tion of the gel would appear to be mutually exclusive. However, it is the temporal-spatial balance of the rate and extent of two stages of s olation, coupled to contraction, that can explain the conversion of ge lled ectoplasm in the tail to a solated endoplasm within the same smal l volume, generation of a force for the retraction of tails, maintenan ce of cell polarity, and creation of a positive hydrostatic pressure t o push against the newly formed endoplasm. The mechanism of solation-c ontraction of cortical cytoplasm may be a general component of the nor mal movement of a variety of amoeboid cells and may also be a componen t of other contractile events such as cytokinesis.