Synchrotron X-ray studies of biopolymers: self-assembly and oscillations of microtubules.

Abstract

Microtubules are fibrous structures that form part of the cytoskeletal network of eukaryotic cells. They are responsible for a variety of cellular functions, for example the static support of a cell (axostyle), movement against the external medium (cilia: directed streaming; flagella: swimming), intracellular transport (e.g., in nerve cells), or the separation of chromosomes during mitosis. Microtubules are hollow cylinders, about 25 nm wide, built of the subunit protein tubulin. There are two forms of tubulin (α and β) of molecular weight 50 kDa. Together they form a heterodimer (α-β) which is the effective assembly unit. A linear string of α-β-heterodimers is called a protofilament; thirteen of these associate laterally to form the microtubule cylinder (Fig. lc, e). A variety of microtubule-associated proteins (MAPs) can be attached to the outside of the microtubule core (Fig. Id). They tend to stabilize microtubules and enhance the efficiency of self-assembly. The mixture of tubulin and MAPs is called microtubule protein (Fig. la). Tubulin and MAPs can be isolated in large quantities from mammalian brain and induced to form microtubules in vitro. This process requires binding and hydrolysis of the nucleotide GTP, and it can be controlled conveniently by temperature (microtubules form at 37°C and fall apart at 4°C). Microtubule protein can assemble into a variety of polymorphic forms, for example, ring-like oligomers are prominent at low temperature (Fig. lb). The rings also contain bound MAPs, and therefore ring-containing solutions assemble more efficiently than purified tubulin. In the past, several assembly models were proposed in which rings were considered as nucleating centers (Kirschner, 1978); the actual mechanism is different, as described below.