Myosin-cell wall interactions during cytokinesis in fission yeast: a framework for understanding plant cytokinesis?

Abstract

Myosins are associated with the division machinery in plants, animals and fungi. In the fission yeast, Schizosaccharomyces pombe, cytokinesis involves a contractile actomyosin ring (CAR). Constriction of the CAR on the inside of the cell membrane is coordinated with the deposition of the septum, the new cell wall that partitions the two newly created daughter cells, on the outside of the cell membrane. S. pombe possesses five myosins, two type II myosins, Myo2 and Myp2, two type V myosins, Myo51 and Myo52, and a single type I myosin, Myo1 (Win et al., 2002). Myo2, Myp2 and Myo51 are components of the cytokinetic actin ring (CAR) while Myo1 and Myo52 are involved in the deposition of the septum. The accumulation of all five myosins at the division plane in M-phase is a function of the septation initiation network (SIN), a signal transduction pathway that couples cytokinesis to mitotic exit. The targets of SIN pathway remain to be defined but probably include Myo2 (Mulvihill et al., 2001) and the 1-3-glucan synthase Cps1 which is involved in the deposition of the septum (Liu et al., 2002). We have developed a new procedure to directly examine the spatial and temporal relationships between CAR formation and contraction and septum formation in living fission yeast cells expressing GFP-tagged Myo2 in combination with the cell wall stain Calcofluor. In wild type cells, CAR contraction is a continuous, smooth process and the centripetal deposition of the septum precisely follows the constricting ring. The dynamics of CAR contraction is identical in cells lacking either Myp2 or Myo51. In the absence of the type V myosin, Myo52, however, CAR contraction is slower and discontinuous. These findings demonstrate the interdependence of CAR contraction and septum deposition and show that Myo52 links the two processes. The class VIII myosins in plants may play a comparable role in the organisation and deposition of new cell wall material at the cell plate (Reichelt et al., 1999). Myo2 and Myp2 have no role other than cytokinesis. The myo2 is an essential gene. Germinating myo2 spores go through a few cell divisions forming short filaments with septa that fail to cleave (May et al., 1997). Cells lacking Myp2 show only a mild cytokinetic defect (Mulvihill et al., 2000). Myo2 and Myp2 contain 1527 and 2104 amino acids, respectively, the difference being entirely due to the length of the -helical tail. The two type II myosins share common essential and regulatory light chains, Cdc4 and Rlc1, respectively. Myo2 is believed to be a conventional two-headed dimer while Myp2 is thought to be monomeric (Bezanilla and Pollard, 2000). The Myo2 tail contains nine proline residues dispersed throughout the structure whereas the Myp2 tail contains 25 prolines, 19 of which are concentrated at the centre, dividing the tail into two subdomains which are thought to fold back on one another to form an anti-parallel coiled coil (Bezanilla and Pollard, 2000). Since we were unable to find an obvious function for Myp2 we wondered whether other fungi that divide by formation of a cytokinetic septum also possessed a second type II myosin. We therefore screened the genome projects of Aspergillus fumigatus, Neurospora crassa for myosin sequences. In each case three myosin genes were found, one each of myosin types I, II and V. This might represent the minimum number of myosins required to sustain cell function eukaryotic cells other than plants which have their own unique myosin inventory (Reddy and Day, 2001). Inspection of the tails of the type II myosins revealed that both were Myp2-like in both length and in the number and disposition of proline residues. A. fumigatus Myo2 contains 2409 amino acids and has 15 prolines within its * Corresponding author. Tel.: +44-20-7679-7343; fax: +44-20-7679-7343. Cell Biology International 27 (2003) 239–240 Cell Biology International