QUICK SEARCH:   [advanced] Author: Keyword(s):
Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents

First published online January 26, 2005
doi: 10.1242/10.1242/jcs.01650

This Article Summary Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Huisman, S. M. Articles by Segal, M. Search for Related Content PubMed PubMed Citation Articles by Huisman, S. M. Articles by Segal, M. Social Bookmarking                         What's this?

Cortical capture of microtubules and spindle polarity in budding yeast - where's the catch?

Department of Genetics, University of Cambridge, Downing Street, Cambridge, CB2 3EH, UK

View larger version (20K): [in a new window]   Fig. 1. Coordinated control of SPB function and cortical capture of astral MTs establishes spindle polarity. MT capture at the prebud primes spindle polarity by directing the old SPB (dark blue) towards the new budding site. Because the SPB is embedded in the nuclear envelope, SPB movement is coupled to the migration of the nucleus towards the bud neck. After SPB duplication, MT organization by the new SPB (light blue) is delayed by Cdk. During SPB separation, a new area of cortical capture at the bud neck stops newly formed MTs from entering the bud. As a result, the new SPB is confined to the mother cell. Already at this stage, the fate of each pole (daughter-bound versus mother-bound) is clearly specified. Correct polarity brings about the orientation of the pre-anaphase spindle along the mother-bud axis. During spindle elongation, the old SPB enters the bud.

View larger version (52K): [in a new window]   Fig. 2. Kar9p-mediated MT delivery to the bud. Kar9p translocates to MT plus ends (away from the SPB) in association with Bim1p and, possibly, Kip2p (not shown). This complex, in turn, interacts with the type V myosin Myo2p to direct bud-ward MT tracking along actin cables organized by the formin Bni1p. Following spindle assembly, Cdc28p-Clb4p is recruited at the daughter-bound pole, to be delivered to the MT plus end in association with Kar9p. This might terminate MT interactions with the distal portion of the bud cell cortex prior to anaphase onset. However, recruitment of Cdc28p-Clb4p does not impart polarity as it is mediated by the asymmetric recruitment of Kar9p to the daughter-bound pole. The molecular basis for controlling this asymmetry remains unclear. Additionally, unbound Kar9p might be transported towards the bud (see text for details).

View larger version (13K): [in a new window]   Fig. 3. Program of MT-cortex interactions set by Bud6p. Accumulation of Bud6p at the incipient bud directs astral MTs from the old SPB (dark blue) to undergo capture at the new bud site. Following bud emergence, interactions continue with the bud cortex decorated by Bud6p. As the spindle assembles, Bud6p begins to concentrate at the bud neck. This causes the old pole to interact with the bud and bud neck cortex whereas the new pole (light blue) is prevented from interacting with the bud. Once polarity is established, the spindle orientates along the mother-bud axis. Bud6p continues to cue cortical interactions during anaphase and following mitotic exit to redirect the SPBs to contact the division site (not shown but see Segal et al., 2002).

View larger version (35K): [in a new window]   Fig. 4. Separate mechanisms for Bud6p and Kar9p in astral MT capture. (A) Kar9p-mediated MT delivery and Bud6p-dependent capture coexist during pre-anaphase spindle orientation. On average, SPBs are associated with three astral MTs but only one is usually bound to Kar9p at any given time in wild-type cells. A Kar9p-bound MT maintains the SPB orientated towards the bud, whereas other MTs undergo Bud6p-dependent capture at the cell cortex to promote SPB movement coupled to MT growth or shrinkage. In addition, Kar9p might affect dynamicity, which explains the processive transport of Kar9p-bound MTs along actin cables and their persistence within the bud. (B) MT dynamic behaviour in association with Kar9p. A Kar9p-bound MT grows (a) and encounters Myo2p (b) to be transported along an actin cable. Transport towards the bud occurs without changes in MT length (b-d). This mode of delivery can be coupled to SPB movement towards the bud (d). (C) MT dynamic behaviour in association with Bud6p cortical sites. A MT encounters cortical Bud6p (a). This results in SPB movement towards the cortex coupled to MT depolymerization (b). Following spindle assembly, this mode of interaction continues to tether the SPB to the bud neck region.

View larger version (28K): [in a new window]   Fig. 5. Dynein-mediated MT sliding couples SPB translocation with spindle elongation. (a) Localization of dynein to MT plus ends depends upon Bik1p (CLIP-170) and Pac1p (LIS1). Bik1p build-up at MT plus ends requires the kinesin Kip2p. Dynein-bound MTs interact with the cortical anchor Num1p to generate pulling force by MT sliding independently of actin (b). Genetic analysis also implicates dynein and Bud6p in promotion of MT shrinkage at the cell cortex, although their role in a common pathway remains to be demonstrated.

CiteULike

Complore

Connotea

Del.icio.us

Digg

Reddit

Technorati

Twitter