Membrane oligosaccharides: structure and function during differentiation.

Abstract

Recent results gathered by normal light microscopy, immunocytochemistry, fluorescent-analog cytochemistry, and electron microscopy have allowed an improved interpretation of ameboid movement and related phenomena. 1. The contractile system responsible in Amoeba proteus for the generation of motive force for protoplasmic streaming and a large variety of dynamic activities is represented mainly by a thin cortical filament layer at the cytoplasmic face of the cell membrane (Fig. 18I). During normal locomotion this layer exhibits a distinct structural and physiological polarity with three different zones: a zone of reformation at the front (A), a zone of contraction in the intermediate cell region (B), and a zone of destruction at the uroid (C). 2. Two types of filaments participate in the formation of the cortical layer: (1) randomly distributed thin (actin) filaments exhibiting a parallel orientation in the anterior (Fc1) and a disordered arrangement in the intermediate and posterior cell region (Fc2; see also Fig. 17b), and (2) thick (myosin) filaments in close association with F-actin and mostly restricted to the intermediate and posterior cell region (Fc2). 3. The internal hydraulic pressure generated by localized active contraction of the cortical layer is transmitted to the endoplasm via the cell membrane and converted into directed streaming by a gel-sol gradient of decreasing viscosity between the uroid and the front. Calcium ions, ATP, and regulative proteins (profilin and a kinase) play an essential role in controlling both the interaction of actin and myosin and the sol-gel state of the cytoplasmic matrix. 4. Any alterations externally induced in the polarity of the cortical filament system by chemical or physical stimulation and inhibition cause immobilization of the amebas (Fig. 18II) with characteristic changes in (1) cell shape (spherulation and cell flattening), (2) membrane dynamics (cytotic and cytokinetic activities), and (3) cytoplasmic organization (hyalogranuloplasmic separation). pseudopodial tip (Fig. 18III, b----c, d----e), (3) destruction of the old layer at the hyalogranuloplasmic border (Fig. 18III, c,e), and (4) alternate solation (Fig. 18III, b and d) and gelation (Fig. 18III, c and e) of the hyaloplasm between the layer and the plasma membrane. The retraction of pseudopodia is accomplished by a local contraction of the cortical layer in conjunction with a simultaneous gel-sol transformation of the ectoplasmic cylinder. 6. The expression of a rather complex cytoskeleton which is composed not only of microfilaments and associated proteins, but also of intermediate- and microtubularlike structures has to be considered in future