Septin-dependent compartmentalization of the endoplasmic reticulum during yeast polarized growth

Abstract

Polarized cells frequently use diffusion barriers to separate plasma membrane domains. It is unknown whether diffusion barriers also compartmentalize intracellular organelles. We used photobleaching techniques to characterize protein diffusion in the yeast endoplasmic reticulum (ER). Although a soluble protein diffused rapidly throughout the ER lumen, diffusion of ER membrane proteins was restricted at the bud neck. Ultrastructural studies and fluorescence microscopy revealed the presence of a ring of smooth ER at the bud neck. This ER domain and the restriction of diffusion for ER membrane proteins through the bud neck depended on septin function. The membrane-associated protein Bud6 localized to the bud neck in a septin-dependent manner and was required to restrict the diffusion of ER membrane proteins. Our results indicate that Bud6 acts downstream of septins to assemble a fence in the ER membrane at the bud neck. Thus, in polarized yeast cells, diffusion barriers compartmentalize the ER and the plasma membrane along parallel lines.

Discreteness-induced concentration inversion in mesoscopic chemical systems

“…44 ), sub-organellar structures in the endoplasmic reticulum (few hundred nanometers 45,46 ) and mitochondria (few to hundred nanometers, 47 ) and inside 'effective' cytoplasmic compartments (35 to 50 nm (ref. 48)) created by molecular sieving effects.…”

A signal comprising a basic cluster and an amphipathic α-helix interacts with lipids and is required for the transport of Ist2 to the yeast cortical ER

“…The cortical ER is a peripheral meshwork of membrane tubules underlying the plasma membrane, with only a few cytoplasmic tubules connecting the cortical ER to the perinuclear ER (Koning et al, 1993;Preuss et al, 1991;Prinz et al, 2000). However, the entire ER is one continuum, as shown by free diffusion of luminal GFP and the mobility of integral ER membrane proteins (Luedeke et al, 2005). The movement of the translocon component Sec61, the SNARE Sec22 and 3-hydroxy-3-methylglutaryl-coenzyme A reductase (Hmg1) between the cortical and perinuclear ER of mother cells is only ten times slower than the free diffusion of GFP in the ER lumen (Luedeke et al, 2005).…”

“…However, the entire ER is one continuum, as shown by free diffusion of luminal GFP and the mobility of integral ER membrane proteins (Luedeke et al, 2005). The movement of the translocon component Sec61, the SNARE Sec22 and 3-hydroxy-3-methylglutaryl-coenzyme A reductase (Hmg1) between the cortical and perinuclear ER of mother cells is only ten times slower than the free diffusion of GFP in the ER lumen (Luedeke et al, 2005). Fluorescence loss in photobleaching experiments in mother cells revealed that half of the Sec61 exchanges between cortical and perinuclear ER in less than 20 seconds.…”

“…Fluorescence loss in photobleaching experiments in mother cells revealed that half of the Sec61 exchanges between cortical and perinuclear ER in less than 20 seconds. Between the ER of mother and daughter cells this exchange occurs within minutes (Luedeke et al, 2005).…”

Structure of the formin-interaction domain of the actin nucleation-promoting factor Bud6

“…In Schizosaccharomyces pombe, the microtubule plus endassociated protein Tea1, formin For3, and Bud6 form a large polarity complex that resides at the cell tips and promotes localized actin cable assembly, and the triple knockout of these three genes leads to severe defects in cell polarity (25). Bud6 also contributes to the maintenance of septin-dependent diffusion barriers in the endoplasmic reticulum and nuclear membranes, which limit membrane protein diffusion between the mother and daughter cell compartments (26,27). The N-terminal half of Bud6 is required for its in vivo localization and for its function in cortical capture of astral microtubule ends (28,29).…”