Stable and Dynamic Axes of Polarity Use Distinct Formin Isoforms in Budding Yeast

Abstract

Bud growth in yeast is guided by myosin-driven delivery of secretory vesicles from the mother cell to the bud. We find transport occurs along two sets of actin cables assembled by two formin isoforms. The Bnr1p formin assembles cables that radiate from the bud neck into the mother, providing a stable mother-bud axis. These cables also depend on septins at the neck and are required for efficient transport from the mother to the bud. The Bni1p formin assembles cables that line the bud cortex and target vesicles to varying locations in the bud. Loss of these cables results in morphological defects as vesicles accumulate at the neck. Assembly of these cables depends on continued polarized secretion, suggesting vesicular transport provides a positive feedback signal for Bni1p activation, possibly by rho-proteins. By coupling different formin isoforms to unique cortical landmarks, yeast uses common cytoskeletal elements to maintain stable and dynamic axes in the same cell. INTRODUCTIONProper cell polarization requires a balance between dynamism and stability. Persistent systems, such as the apical and basolateral domains of epithelial cells, show a higher stability than flexible systems, such as cells undergoing chemotaxis. However, both types can use similar cytoskeletal components, so an important task in understanding the control of polarity is to identify features that influence persistence, and how those features integrate with the core cytoskeletal machinery providing the physical expression of polarity.The process of bud growth of the yeast Saccharomyces cerevisiae provides a model for examining the control of polarity (for reviews, see Bretscher, 2003;Chang and Peter, 2003). Secretory organelles, such as the Golgi and endoplasmic reticulum, are distributed throughout the bud and mother cell, whereas post-Golgi secretory vesicles concentrate at discrete growth sites at the cell cortex. These vesicles are guided to these sites along what are essentially two axes of polarity: a stable axis directing traffic from the mother into the bud, and a dynamic axis that fine-tunes delivery from the bud neck to specific regions in the bud, sending vesicles first to the small bud tip, then to the entire bud surface, and finally back toward the neck during mother/daughter separation. On delivery to these locations, post-Golgi vesicles fuse with the cell surface to promote localized cell expansion.Two class-V myosins, with heavy chains encoded by MYO2 (Johnston et al., 1991) and MYO4 (Haarer et al., 1994), propel cellular components, including vesicles, organelles, mRNAs, and microtubules, from the mother into the bud during growth and organelle segregation. The myosins move along cables of actin filaments that radiate throughout the cell in arrays oriented toward growth sites (Adams and Pringle, 1984;Kilmartin and Adams, 1984).Assembly of these cables depends on two formin homologues, Bni1p and Bnr1p (Evangelista et al., 2002;Sagot et al., 2002a), members of a family of cytoskeletal regulatory proteins defined by conserved fo...

Membrane Organization and Dynamics in Cell Polarity

“…It was shown that although depolymerization of all F-actin prevented polarization of most proteins at the bud tip, it did not prevent polarization of Cdc42 (Ayscough et al 1997;Irazoqui et al 2003). It was later shown that selective depolymerization of actin cables, but not actin patches, caused dispersal of Cdc42 from the bud tip (Irazoqui et al 2005;Pruyne et al 2004b). Actin patches are associated with sites of active endocytosis.…”

From Function to Shape: A Novel Role of a Formin in Morphogenesis of the FungusAshbya gossypii

“…Although S. pombe for3⌬ had no visible actin cables (Feierbach and Chang, 2001) or the cables were shorter in size and reduced in number (Nakano et al, 2002), cells were still able to grow in a polarized manner unlike a deletion of Agbni1. Different functions were also described for the baker's yeast formins ScBni1p and ScBnr1p (Pruyne et al, 2004). ScBni1p localizes mainly to the bud tip where it forms actin cables.…”

“…In S. cerevisiae, the two formins Bni1p and Bnr1p are required for cell polarity and cytokinesis with some overlapping and some different functions (Zahner et al, 1996;Evangelista et al, 1997;Imamura et al, 1997;Kamei et al, 1998;Pruyne et al, 2004). In Schizosaccharomyces pombe, three formins exist which are also involved in cell polarity and cytokinesis.…”

Deconstructing formin-dependent actin cable assembly

“…Little Bud6 is recovered in extracts of wild-type cells, so it would be very interesting to supplement the extracts with additional Bud6 protein and maybe thereby drive assembly in nonmitotic extracts. Another limitation is that no cable assembly was seen when the FH1-COOH region of Bnr1 was used, although this region can nucleate and elongate filaments, and cells expressing just Bnr1 have actin cables (19). To address these issues, it is likely that the method used for preparing the extract may need to be adjusted, perhaps increasing its concentration, or adding an ATP regenerating system, or "spiking" it with additional purified yeast actin.…”