Tubulin-dynein system in flagellar and ciliary movement

Abstract

Eukaryotic flagella and cilia have attracted the attention of many researchers over the last century, since they are highly arranged organelles and show sophisticated bending movements. Two important cytoskeletal and motor proteins, tubulin and dynein, were first found and described in flagella and cilia. Half a century has passed since the discovery of these two proteins, and much information has been accumulated on their molecular structures and their roles in the mechanism of microtubule sliding, as well as on the architecture, the mechanism of bending movement and the regulation and signal transduction in flagella and cilia. Historical background and the recent advance in this field are described.

Figure 1.

Schematic cross-sections of flagellum and cilium in protists and lower plants (left) and in choanoflagellates and animals (right), modified from Ref. .

Figure 2.

Structure and in situ configuration of outer arm dynein. (a) Schematic subunit structure of outer arm dynein from Ciona sperm flagella. The structure is divided into three parts; head, stalk and stem. Outer arm dynein is composed of two heavy chains (α and β), five intermediate chains (ICs) and six light chains (LCs). (b) Longitudinal configuration of outer arm dynein bound to doublet microtubules. (c) Structure of the outer doublet microtubule and in situ configuration of outer arm dynein. The outer and inner junction (OJ and IJ) connect the A- and B-tubules. Microtubule inner proteins (MIP) and the “beak” structure are associated with the internal wall of the A- or B-tubule. Note that the Ciona α or β heavy chain corresponds to the sea urchin β or α and Chlamydomonas β/α or γ, respectively.

Figure 3.

Relationship between beat frequency and maximum shear angle. The flagellar beating of activated (●), transient (□) and hyperactivated (■) golden hamster spermatozoa is shown. The line is a least squares regression line, given by f = 14/θ_max, where f is the beat frequency and θ_max is the maximum shear angle. Since the sliding velocity is proportional to f·θ_max, this equation clearly shows the constant rate of microtubule sliding before and after hyperactivation.

Figure 4.

Maturation and capacitation of mammalian spermatozoa. The flagellar movement of intact (1–3) and demembranated and ATP-reactivated (1′–3′) testicular, caput epididymal and cauda epididymal golden hamster spermatozoa are shown, respectively. Panels 4 and 5 represent the intact hyperactivated and the intact hyperactivated and acrosome-reacted spermatozoa, respectively. The time indicates the intervals between successive tracings.