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© The Rockefeller University Press,
0021-9525/1999//809 $5.00
The Journal of Cell Biology, Volume 144, Number 5,
, 1999 809-811
Mini-Review |
Formin' the Connection between Microtubules and the Cell Cortex
IN higher eukaryotes, the position of cytokinesis is determined by the position of the mitotic spindle (Rappaport, 1990). Generally, the spindle sits in the middle of the cell and cytokinesis produces two equivalent cells. However, many developmental processes require specific positioning of the cleavage plane and, hence, the mitotic spindle. For example, control of spindle position can be used to asymmetrically distribute cell fate determinants between the two daughter cells, to form polar bodies during oogenesis, and for tissue morphogenesis (Stearns, 1997). Positioning of the spindle appears to be mediated through the attachment of astral microtubules to filamentous actin at the cell cortex (Lutz et al., 1988; Hyman, 1989; Waddle et al., 1994).
In budding yeast, the site of cell division is specified at the start of the cell cycle by the location of the bud site. The mitotic spindle must then be positioned in the neck between mother and bud to achieve segregation of chromosomes between the mother and daughter cells. Positioning the mitotic spindle in yeast involves three processes (Fig. 1, Table I). Before mitotic spindle formation, a kinesin, Kip3p, is required for movement of the nucleus to the mother-bud neck (DeZwaan et al., 1997). After spindle formation, Kip3p is needed to orient the pre-anaphase spindle along the mother bud axis. Dynein functions later in mitosis to move the anaphase spindle into the neck (Stearns, 1997). How Kip3p and dynein function in these movements is not known. They may pull on microtubules in the traditional sense of a motor, or they may regulate microtubule length via effects on dynamic instability at microtubule ends. Astral microtubules are highly dynamic and occasionally span the distance from the spindle pole body to the cell cortex (Shaw et al., 1997). Therefore, spindle movements are hypothesized to depend on transient, short-lived interactions between astral microtubules and the cell cortex. Two papers in this issue of the Journal define components involved in microtubule-cortex interactions during the early spindle movements that require Kip3p (Lee et al., 1999; Miller et al., 1999).
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Bni1p, a Formin, Functions in Kip3p-dependent Spindle Positioning and Movement |
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 Top Bni1p, a Formin, Functions... Bni1p Participates in Kar9p...A Role for Filamentous...Nuclear Positioning during...ConclusionsFuture DirectionsReferences |
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These phenotypes are similar to ones observed previously in kip3 and kar9 mutants (Cottingham and Hoyt, 1997; DeZwaan et al., 1997; Miller and Rose, 1998; Miller et al., 1998). Previous genetic analyses suggested that Kip3p and Kar9p act together to position the spindle before the action of dynein (Miller et al., 1998). Genetic analyses in the new reports (Lee et al., 1999; Miller et al., 1999) indicate that Bni1p functions in the same process as Kip3p and Kar9p (Fig. 1). Bud6p, a protein that physically interacts with Bni1p, has a similar but less important role, based on milder phenotypes (Lee et al., 1999; Miller et al., 1999) and a weaker genetic interaction with dynein (Miller et al., 1999).
Interestingly, bni1 mutants do display movements of the pre-anaphase spindle, including exaggerated transits back-and-forth through the neck. Therefore, alternative mechanisms for movement may exist, and Bni1p may act as a governor to focus or restrict the action of these other mechanisms.
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Bni1p Participates in Kar9p Localization |
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TopBni1p, a Formin, Functions...Bni1p Participates in Kar9p...A Role for Filamentous...Nuclear Positioning during...ConclusionsFuture DirectionsReferences |
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These results suggest that Bni1p and Bud6p localize Kar9p to the cortex, and that the mislocalization of Kar9p in bni1 and bud6 mutants is responsible for the nuclear positioning and pre-anaphase spindle orientation defects in these mutants. However, microtubules were appropriately oriented in the bni1 mutant, and the spindle positioning defects in kar9 mutants were more severe than the defects in bni1 mutants. Therefore, some Kar9p function appears to be retained despite its mislocalization in bni1 mutants.
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A Role for Filamentous Actin |
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TopBni1p, a Formin, Functions...Bni1p Participates in Kar9p...A Role for Filamentous...Nuclear Positioning during...ConclusionsFuture DirectionsReferences |
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What element of the actin cytoskeleton provides this function? Cortical actin patches have been widely assumed to be the attachment site for microtubules because the patches cluster at the bud tip. However, clustering of actin patches may not be necessary for pre-anaphase spindle orientation. An actin cytoskeleton mutant with largely delocalized patches, sla1
SH3#3, showed normal spindle orientation and positioning (Lee et al., 1999). Also, Kar9p localized normally in a sla1 null mutant (Miller et al., 1999). In a similar analysis, the bipolar pattern of bud site selection in diploid yeast depended on actin but not patches (Yang et al., 1997). Furthermore, several proteins are involved in both bipolar bud site selection and spindle orientation. Thus, both processes may involve some as yet undefined form of filamentous actin; alternatively, a small amount of actin patch clustering may be sufficient.
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Nuclear Positioning during Mating |
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TopBni1p, a Formin, Functions...Bni1p Participates in Kar9p...A Role for Filamentous...Nuclear Positioning during...ConclusionsFuture DirectionsReferences |
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In shmoos, bni1 and bud6 mutations impaired Kar9p localization, astral microtubule orientation and nuclear movement into the projection. The extent of Kar9p mislocalization correlated with the severity of the defects in microtubule orientation and nuclear movement. However, the phenotypes in shmoos were more severe than those in dividing cells. Therefore, mating may provide a simpler model for cortical capture of astral microtubules.
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Conclusions |
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TopBni1p, a Formin, Functions...Bni1p Participates in Kar9p...A Role for Filamentous...Nuclear Positioning during...ConclusionsFuture DirectionsReferences |
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Bni1p, actin and Kar9p are all necessary for the early phases of spindle positioning and orientation, which depend on astral microtubules and Kip3p, a kinesin. Bni1p and actin function together to localize Kar9p.
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Future Directions |
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TopBni1p, a Formin, Functions...Bni1p Participates in Kar9p...A Role for Filamentous...Nuclear Positioning during...ConclusionsFuture DirectionsReferences |
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The mechanism of force production to move the spindle is unknown. The kinesin Kip3p is presumably involved, but whether Kip3 functions as a microtubule motor or causes microtubule shortening by destabilizing ends is an important open question.
Whether this microtubule/cortex attachment mechanism operates outside of yeast is also unknown. Formins, such as Bni1p, are found in many different organisms. Formins appear to influence the actin cytoskeleton but have not yet been implicated in interactions between actin and microtubules or been shown to have primary effects on microtubules. Kar9p has no obvious homologues in the sequence databases. Studies of formins and associated proteins, including perhaps functional equivalents of Kar9p, in other systems will be important.
In addition, little is known about how microtubules attach to the cell cortex during the dynein-mediated movement of the spindle into the neck in yeast. Dynein-dependent spindle movements are known to occur in organisms other than yeast (Morris et al., 1995; Skop and White, 1998).
A submitted manuscript by K. Bloom and colleagues (Yang, C., P. Maddox, E. Chin, E. Yeh, E.D. Salmon, D.J. Lew, and K. Bloom, manuscript submitted for publication). Cytoplasmic dynein, Bni1p and Bud6p are required for mitotic spindle orientation and nuclear migration in yeast) describes results consistent with those discussed here.
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Acknowledgments |
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Submitted: 11 February 1999
Revised: 16 February 1999
Address correspondence to John Cooper, Box 8228, 660 S. Euclid Ave., St. Louis, MO 63110. Tel.: (314) 362-3964. Fax: (314) 362-0098. E-mail: jcooper @cellbio.wustl.edu
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References |
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TopBni1p, a Formin, Functions...Bni1p Participates in Kar9p...A Role for Filamentous...Nuclear Positioning during...ConclusionsFuture DirectionsReferences |
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