Cytokinesis in yeast and animal cells depends on extensive remodeling of the cell membrane at the cleavage furrow. The inward movement of the furrow generally results in an increase in the surface area of the cell membrane, and the additional membrane seems to be provided by the insertion of new membrane at the cleavage site. In yeast, new cell wall materials are also deposited at this site, resulting in the process of septation – the formation of a new cell wall, or septum, between the daughter cells.

Addition of new membrane occurs by the fusion of membrane vesicles with the plasma membrane near the inner edge of the cleavage furrow. These small vesicles originate in the Golgi apparatus and are targeted to the plasma membrane at the cleavage site by components of the secretory apparatus, including members of the syntaxin family of vesicle-targeting proteins.

In most cell types, microtubules provide the tracks along which membrane vesicles are transported to the site of cleavage. The importance of microtubules for membrane delivery is particularly apparent in the cells of higher plants, in which cytokinesis is entirely a process of membrane and wall deposition – without any need for a contractile ring. Cytokinesis in these cells is directed by an organelle called the phragmoplast, an array of microtubules, derived from the anaphase spindle, whose plus ends are embedded in a protein matrix along the cell midline (Figure 8-8). Membrane vesicles carrying the raw materials for cell wall synthesis travel along these microtubules to the center of the phragmoplast, where they promote the deposition of a new membrane and cell wall to form the cell plate. Unlike the case in yeast and animal cells, where all new membrane is added to the preexisting cell membrane, new membrane formation in plants starts in the center of the cell and spreads outward until it meets the plasma membrane.

The importance of microtubules in membrane addition is also illustrated by studies of Xenopus embryonic cells. These cells contain a specialized microtubule array, called the furrow microtubule array, that forms at the inner edge of the cleavage furrow (Figure 8-9). It is required for membrane addition at the furrow and provides the tracks along which membrane vesicles are carried to the site of membrane fusion. The furrow microtubule array may be a specialized structure for enhancing membrane addition in the large and rapidly dividing cells of animal embryos, in which the spindle is often quite distant from the cell membrane. In smaller and more slowly dividing somatic cells, astral microtubules and microtubules of the central spindle are thought to provide a similar function.

Deposition of new membrane in cytokinesis is less extensive in yeast, in which the amount of new membrane needed represents a small fraction of total membrane surface area. In budding yeast, for example, most new membrane addition occurs throughout the cell cycle in the growing bud, and only a small amount is required at the bud neck in the final stages of division (Figure 8-10). In these cells, as in those of metazoans, new membrane is provided by Golgi-derived vesicles. Unlike in metazoans, however, vesicle transport to the bud neck does not require microtubules but instead depends on myosin-dependent transport along actin filaments.

Transport of vesicles to the cleavage site also delivers proteins that help organize the contractile ring and serve other functions in cell separation. In budding yeast, for example, Golgi-derived vesicles that fuse with the bud-neck membrane contain the transmembrane enzyme chitin synthase, Chs2, which synthesizes chitin, the complex polysaccharide that forms the primary septum between mother and daughter cells (see Figure 8-10).

The connection between daughter cells is finally severed when the inwardly moving membranes contact and fuse with each other. This poorly understood process requires the removal of the contractile ring and, in animal cells, depends on the construction of a large protein complex called the midbody at the division site. The final membrane fusion event depends on machinery like that involved in membrane fusion events in the secretory pathway.

Membrane deposition generally occurs in parallel with the contraction of the actin–myosin ring, so that new membrane is added at a rate that matches the rate at which the ring moves inward. It is not clear how membrane addition and ring contraction are coordinated with each other. One simple possibility is that the two processes are independent but occur in parallel because they are both triggered by the same upstream regulatory mechanism. Dephosphorylation of Cdk targets, for example, may be an important mechanism for initiating both processes.

Another possibility is that the membrane deposition machinery and the contraction machinery are physically coupled in some way, so that the progression of each process depends on the other. There is good evidence for this in both budding and fission yeasts, in which inhibition of membrane vesicle delivery or cell wall synthesis causes defects in ring contraction. In animal cells, however, there is less evidence for direct coupling, and it seems that membrane addition and ring contraction are at least partly independent. Inhibition of actin polymerization or Rho activation, for example, blocks the formation of the contractile ring in Xenopus embryonic cells but does not affect membrane insertion. Conversely, inhibition of membrane insertion (by depolymerization of microtubules in the furrow microtubule array, for example) does not prevent contractile ring formation and the initiation of contraction – although full inward contraction of the ring cannot occur without the addition of new membrane.

midbody: large protein complex, derived from the spindle midzone, that is involved in the final stages of cell separation in dividing animal cells.

phragmoplast: organelle in a dividing plant cell upon which the new cell membranes and cell walls between the two daughter cells are constructed. It corresponds to the central spindle of animal cells.

septum: the extracellular wall that forms between two daughter cells in fungi during cell division.

Figure 8-9a Microtubule behavior in the cleaving Xenopus embryo. Photograph kindly provided by Michael Danilchik and Kay Larkin. Reprinted from Dev. Biol., Volume 194, Danilchik, M.V., Funk, W.C., Brown, E.E. and Larkin, K.: Requirement for microtubules in new membrane formation during cytokinesis of Xenopus embryos, Pages 47–60, ©1998, with permission from Elsevier.