Polarity establishmentin yeastJavier E. Irazoqui 1 and Daniel J.Lew1,*Department of Pharmacology and Cancer Biology,Duke University Medical Centre, Durham, NC27710, USA

*Author for correspondence (e-mail:daniel.lew@duke.edu)

Journal of Cell Science 117, 2169-2171 Published byThe Company of Biologists 2004doi:10.1242/jcs.00953

The unicellular yeast Saccharomycescerevisiae is a model system for theestablishment of cell polarity. Yeastcells proliferate by budding, whichinvolves specialization of a small patchof the mother cell cortex andpolarization of many cell constituentstowards that patch, promoting growth of

the bud. Genetic approaches haveuncovered a large number of genesinvolved in these processes, yieldinga working model for polarityestablishment.

Polarized structuresThe poster depicts a yeast mother cellshortly before the emergence of a bud.A large number of proteins becomeconcentrated in a polarized patch about0.5 µm diameter. A ring of cytoskeletalfilaments called septins surrounds thepatch, and actin cables course through thecytoplasm, terminating near the patch.The organization of septin filamentswithin the ring is unknown. Although theactin cables are depicted as singlefilaments for simplicity, in fact eachcable is thought to consist of manyshorter filaments linked into parallel

bundles, with each filament oriented sothat the barbed (plus) end points towardsthe polarized patch. Myosin motors(Myo2p and Myo4p) travel along thecables towards the patch, transportingmany types of cargo, including secretoryvesicles, various organelles, the plus endsof cytoplasmic microtubules and RNA-protein complexes. Mitochondria are alsotransported along cables, though perhapswithout need for myosin motors.Polarized secretion of vesicles carryingcell wall remodeling enzymes and newcell wall constituents promotes local cellwall deformation and bud emergence.Trafficking of other cargos serves tosegregate organelles into the growingbud and to orient the microtubule spindlealong the mother-bud axis. Not depictedare cortical actin patches, mobile short-lived structures linked to endocytosis,which are ‘born’ near the polarized patch.

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(See poster insert)

Javier E. Irazoqui and Daniel J. Lew

Journal of Cell Science 2004 (117, pp. 2169-2171)

Polarized patch

Septin ring

Myo2p

Actin cable

Late Golgi

Cell wall

Cdc55p

Bud14p

Ste20p

Bud5p

Pea2p Msb3p

Bem2p Cla4p

Tos2p Bud6p

Yhr149cp

Cdc42p Gic1p

Spa2p

Boi1p

Kel1p

Kel2p

Zds1p

Tpd3p

Rsr1p

Fks1p

Bud8p

Sec4p

Rga1p

Cdc24p

Msb4p

Boi2p

Rga2p

Bem3p

Bem1p

Bni1p

Msb2p

Gic2p Rom2p

Slg1p

Gsc2p

Sph1p

Rho1p

Mkk2p

Slt2p

Cwh43p

Cbk1p

Kic1p

Hym1p

Tao3p Sog2p

Pkc1p

Mkk1p

Mob2p

Cmd1p Myo2p

Smy1p

Myo4p

She3p

Mlc1p Ypt11p

Sec1p

Exo70p

Sec8p Sec5p

Exo84p

Axl2p

Msb1p

Sec6p

Sec3p Sec15p

Sec10p

Polarity establishment

Bud site selection

Exocyst

Polarized secretion

Polarisome

MAPK signaling

RAM

Cellwall

Rho -GAPs

Myo4p

Vacuole

Microtubule

Bud2p

Peroxisome

Secretoryvesicle

MitochondrionASH1 mRNA

S

G2

M

G1

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Polarization during the cell cycleThe cell cycle of budding yeast isdepicted in the cell interior. Cell cyclecommitment in late G1 phase triggersthe assembly of the polarized patch (red)and septin ring (green), as well as thepolarization of actin cables and corticalactin patches (not shown). Polarizedsecretion leads to bud emergence (atabout the time of the G1/S transition)and then bud growth. The septin ringspreads to form an hourglass-shapedcollar in the cell cortex at the neck,remaining there throughout bud growthand acting as a diffusion barrier thatprevents the mixing of integral andperipheral membrane proteins betweenthe mother cortex and the bud cortex.Early on during bud growth thepolarized patch remains focused at thebud tip, but then becomes broader anddissipates as the bud enlarges. Followingmitosis most of the proteins of thepolarized patch reassemble at themother-bud neck and the septinhourglass splits into two rings duringcytokinesis.

Components of the polarizedpatchAll of the proteins known to localizewithin the polarized patch are depictedin the network at the center of the figure,derived using Osprey software from datacurated at the Saccharomycesgenomedatabase (http://genome-www.stanford.edu/Saccharomyces/) and edited by theauthors. Each protein is marked by acircle, and reported protein-proteininteractions among this set are shown asconnecting lines between the circles(these include reports of two-hybrid andco-immunoprecipitation data as well asdirect interactions among recombinantproteins). Interacting proteins not knownto localize to the patch were excluded.The proteins are divided into color-coded functional groupings.

Bud site selection The location of the polarized patch is notrandom within the cell. Newborn cellscarry spatial landmarks (includingBud8p at the distal tip) that influence thelocation of the polarized patch in thesubsequent cell cycle. The Rsr1pGTPase and its guanine nucleotideexchange factor (GEF) Bud5p and

GTPase-activating protein (GAP) Bud2pare required for proper localization ofthe polarized patch to the site specifiedby the spatial landmarks, and in theirabsence the polarized patch forms at arandom location.

Polarity establishmentThe Rho-family GTPase Cdc42p and itsGEF Cdc24p are essential for assemblyof the polarized patch and septin ring,and for the polarization of actin cablesand cortical actin patches. As such, theyare considered the master regulators ofpolarity establishment in yeast. Geneticstudies indicate that the Cdc42p effectorsCla4p, Ste20p, Gic1p and Gic2p, as wellas the scaffold proteins Bem1p, Boi1pand Boi2p act together with Cdc42p andCdc24p to establish polarity.

Rho-GAPsAt least four GAPs can stimulate GTPhydrolysis by Cdc42p. Phenotypicanalysis suggests that Bem2p isimportant for polarity establishment,although it also has links to Rho1p andthe control of cell wall integrity andMAPK signaling. By contrast, Rga1p,Rga2p and Bem3p have been implicatedin promoting the assembly of the septinring around the polarized patch.

PolarisomeThe polarisome is a protein complexthought to form a link between polarityestablishment factors and actin cables.The formin Bni1p promotes nucleationand growth of actin cables, and Spa2pand Bud6p are important for Bni1plocalization and function. Although actinpolarization is essential for bud growth,the polarisome components are notessential, perhaps because anotherformin, Bnr1p, is recruited to the septinring and can nucleate polarized actincables from there.

Polarized secretionThe type V myosin Myo2p, withassociated light chains Mlc1p andCmd1p (calmodulin), transportssecretory vesicles containing the Rab-family GTPase Sec4p to the polarizedpatch. The SNARE-binding proteinSec1p is also polarized and essential

for vesicle fusion with the plasmamembrane. Myo2p also transportsorganelles and microtubules along actincables, while the related Myo4ptransports mRNA-protein complexes,generating mother-bud differences inprotein translation.

ExocystThe exocyst is a multiprotein complexthat tethers secretory vesicles to theplasma membrane prior to fusion.

Cell wallGlucan polymers make up a largeportion of the cell wall, and the glucansynthases Fks1p and Gcs2p extrude thepolymers across the plasma membrane atsites of cell growth. A cell wall protein(Cwh43p) and a putative sensor of cellwall stress (Slg1p) are also polarized, asis the Rho1p GEF Rom2p. Rho1p is amultifunctional GTPase that activatesglucan synthase as well as the proteinkinase C Pkc1p.

MAPK signalingCell wall stress activates the ‘cellintegrity’ MAPK signaling cascade,several members of which are found inthe polarized patch (Rho1p, Pkc1p,Mkk1p, Mkk2p and the MAPK Slt2p).This pathway activates transcription ofcell-wall-related genes and contributesto halting of the cell cycle underconditions of stress, through themorphogenesis checkpoint.

RAMA recently identified signaling pathwaytermed the RAM (regulation of Ace2pactivity and cellular morphogenesis)contains interacting componentsrequired for optimal polarization as wellas asymmetric mother/daughter geneexpression. During bud growth theseproteins are localized to the polarizedpatch, but some components relocate tothe daughter cell nucleus in the budfollowing mitosis.

A hierarchical model for cellpolarizationThe transition of a yeast cell from anunpolarized state (where only the bud

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site selection proteins are spatiallyrestricted) to the polarized state depictedin the poster involves the near-simultaneous polarization of all ofthe structures discussed above.Examination of whether specificproteins or structures can becomepolarized in the absence of others hasled to a hierarchical model for cellpolarization. In response to a cell cyclecue, Cdc42p together with a subset ofpolarized patch proteins clusters into apatch at a location usually designated bythe bud site selection landmarks. Theseproteins promote the independentassembly of the septin ring and the actincables (and possibly also the actinpatches). The septins then recruit a hostof proteins to the ring, and the cablesdeliver more cargo, such as proteins andorganelles, to the patch. Although thismodel accounts for most known aspects

of polarization, there is probably areinforcing cross-talk among thesestructures once they are polarized. Forinstance, the recruitment of the forminBnr1p to the septin ring might reinforcepolarized actin cable assembly, andpolarized patch factors includingRho1p, Bud6p, and even Cdc42p itselfmight be delivered to the patch onsecretory vesicles traveling along actincables.

Recommended readingPringle, J. R., Bi, E., Harkins, H. A., Zahner, J.E., De Virgilio, C., Chant, J., Corrado, K. andFares, H. (1995). Establishment of cell polarity inyeast. Cold Spring Harbor Symp. Quant. Biol.60,729-744.Chant, J. (1999). Cell polarity in yeast. Annu. Rev.Cell Dev. Biol.15, 365-391.Pruyne, D. and Bretscher, A. (2000a).Polarization of cell growth in yeast. J. Cell Sci.113, 365-375.

Pruyne, D. and Bretscher, A. (2000b).Polarization of cell growth in yeast. J. Cell Sci.113, 571-585.Gladfelter, A. S., Pringle, J. R. and Lew, D. J.(2001). The septin cortex at the yeast mother-budneck. Curr. Opin. Microbiol.4, 681-689.Schott, D., Huffaker, T. and Bretscher, A.(2002). Microfilaments and microtubules: the newsfrom yeast. Curr. Opin. Microbiol.5, 564-574.Fehrenbacher, K. L.,. Boldogh, I. R and Pon, L.A. (2003). Taking the A-train: actin-based forcegenerators and organelle targeting. Trends CellBiol. 13, 472-477.Longtine, M. S. and Bi, E.(2003). Regulation ofseptin organization and function in yeast. TrendsCell Biol. 13, 403-409.

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