Electron microscopy of microtubules decorated with motor protein heads indicate a "3 start helix" in which each turn of the helix spans 3 tubulin monomers (e.g., a, b, a). This results in the microtubule wall having a "seam" where, instead of the predominant aa and bb lateral contacts, a subunits are laterally adjacent to b subunits. 

During in vitro microtubule assembly, tubulin heterodimers join end-to-end to form protofilaments. These associate laterally to form sheets, and eventually microtubules.

In vitro, heterodimers can add or dissociate at either end of a microtubule, but there is greater tendency for subunits to add at the plus end, where b-tubulin is exposed. As with actin filaments, microtubules can undergo treadmilling, with addition of tubulin heterodimers at the plus end and dissociation of tubulin heterodimers at the minus end.

The minus end of a-tubulin may contribute an essential residue to the catalytic site of b-tubulin. Thus the minus end of an a subunit may serve as GAP (GTPase activating protein) for b-tubulin of the adjacent dimer in a protofilament. 

Each nucleotide in the tubulin protofilament is at an a-b interface. The inability of GTP to dissociate from the a-subunit is consistent with occlusion by a loop from the b subunit. A similar occlusion would account for the inability of b-tubulin within a protofilament to exchange bound GDP for GTP.

The nucleotide-binding domain of tubulins includes a highly conserved sequence GGGTG(T/S)G. This sequence, shown in black at right, is part of a loop and helix that extends from one of the b-strands (colored magenta) and passes near the nucleotide phosphates. 

The A tubule of a doublet or triplet microtubule is a complete cylinder, made of 13 protofilaments. "Piggyback" B or C tubules are made of less than 13 protofilaments, usually 10. Micrographs in A p. 966, 968.

Centriolar microtubules are relatively stable. The a,b tubulin heterodimers present in centriolar triplet microtubules are modified by polyglutamylation. Additional tubulins, designated d, e, z & h, as well as other proteins, are either present in centrioles or required for their formation. There is some variability in composition among different organisms.

Basal bodies of cilia and flagella are also centrioles. See electron micrograph in an article by J. Beisson & M. Wright (requires subscription to Current Opinion in Cell Biology).

• The 2 centriole cylinders separate.
• A daughter centriole grows from a short disk-like structure at right angles to each parent centriole.
• A line through each daughter centriole bisects the parent.

(Biogenesis of ciliary basal bodies, which is somewhat different, will not be discussed here.)

The centrosome, a mass of protein also called the microtubule organizing center (MTOC) or pericentriolar material, surrounds centrioles in animal cells. Following duplication, the pericentriolar material initially is associated only with each parent centriole cylinder. 

g-Tubulin, which is homologous to a and b tubulins, nucleates microtubule assembly within the centrosome. Several (12-14) copies of g-tubulin associate in a complex with other proteins called "grips" (gamma ring proteins). This g-tubulin ring complex is seen by electron microscopy to have an open ring-like structure resembling a lock washer, capped on one side.

Microtubules nucleated by the g-tubulin ring complex appear capped at one end, assumed from other data to be the minus end. Polymerization at the minus end of these microtubules is inhibited. Grip proteins of the cap may be involved in mediating binding to the centrosome.

Phosphorylation of a conserved tyrosine residue of g-tubulin has been shown to regulate microtubule nucleation in yeast cells.

During interphase, the centrosome (MTOC) is usually located near the nucleus. Microtubules grow out from the MTOC, forming a hub and spoke array, even during interphase. See A p. 930-932.

With minus ends of most microtubules anchored in the centrosome, microtubules grow and shrink mainly through addition and loss of tubulin heterodimers at their plus ends.

A sub-population of microtubules with free minus ends exists in some cells. These may arise by breakage or cleavage of microtubules.

In vitro, the tendency to grow or shrink may be a function of tubulin concentration. As microtubules grow, tubulin dimers are depleted. Below a critical tubulin concentration, rapid shrinkage at the plus end has been attributed to loss of a "GTP cap." Hydrolysis of GTP by b-tubulin, as polymerization brings it into contact with a-tubulin, takes time. A rapidly growing microtubule may accumulate a few layers of tubulin-GTP at the plus end. 

Fraying or curving of protofilaments is observed at the ends of rapidly disassembling microtubules. This may be due to a change in conformation when b-subunits at the plus end have bound GDP instead of GTP. Tubulin heterodimers with GDP bound to the b subunit form ring shaped assemblies in vitro. Straight protofilaments form only when both tubulins have bound GTP. Diagrams & micrographs in A. p. 919.

Dynamic instability of microtubules in vivo is regulated by interaction with other proteins. For example, during prophase of mitosis, microtubules grow out from the centrosome. If the plus end of a microtubule makes contact with a chromosome, it becomes stabilized. Otherwise rapid disassembly at the plus end ensues, and the tubulin dimers are available for growth of another microtubule.

• For example, MCAK, a member of the kinesin family of proteins, promotes dissociation of tubulin dimers at the kinetochore, as kinetochore-linked microtubules shorten during anaphase of mitosis.
  In vitro, MCAK can cause microtubules to shrink at both ends.
  See a movie showing shortening of fluorescent-labeled microtubules after addition of MCAK (video supplement #1 from article by Helenius et al.; requires a subscription to Nature).

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