Mechanisms of Intracellular Organelle Transport

Abstract

All eukaryotic cells exhibit some form of intracellular motility. The spectrum of motile activities observed inside living cells is extraordinarily broad, ranging from processes barely detectable even in time-lapse recordings, to the breathtaking bulk transport of cytoplasm best observed by slow-motion analysis. On the basis of their phenomenology, motile activities of cytoplasmic constituents may be subdivided into three broad categories: 1. Bulk movement of cytoplasm in association with, or as a consequence of, cell deformation. Good examples include the extension and retraction of cell processes (pseudopodia) in amoeboid cells, an activity accompanied by flow of cytoplasm into or out of these cell extensions. Cellular inclusions are carried along in the cytoplasm in a seemingly passive manner. Organelle translocation ceases as the protrusive or retractive activity of the cell comes to a standstill. 2. Uniform, continuous transport of organelles and cytoplasm along more or less defined pathways in the absence of cell deformation. Prime examples include the rotational or vectorial “streaming” of endoplasm in many plant cells, and shuttle streaming in slime molds. 3. Discontinuous, erratic, “saltatory” movements of particles and organelles. This form of transport is observed in a wide variety of eukaryotic cells and includes such diverse phenomena as organelle movements in protists and fast transport of materials in neurons.