The Sur7p Family Defines Novel Cortical Domains in Saccharomyces cerevisiae, Affects Sphingolipid Metabolism, and Is Involved in Sporulation

Abstract

We have discovered a novel cortical patch structure in Saccharomyces cerevisiae defined by a family of integral plasma membrane proteins, including Sur7p, Ynl194p, and Ydl222p. Sur7p-family patches localized as cortical patches that were immobile and stable. These patches were polarized to regions of the cell with a mature cell wall; they were absent from small buds and the tips of many medium-sized buds. These patches were distinct from other known cortical structures. Digestion of the cell wall caused Sur7p patches to disassemble, indicating that Sur7p requires cell wall-dependent extracellular interactions for its localization as patches. sur7⌬, ydl222⌬, and ynl194⌬ mutants had reduced sporulation efficiencies. SUR7 was originally described as a multicopy suppressor of rvs167, whose product is an actin patch component. This suppression is probably mediated by sphingolipids, since deletion of SUR7, YDL222, and YNL194 altered the sphingolipid content of the yeast plasma membrane, and other SUR genes suppress rvs167 via effects on sphingolipid synthesis. In particular, the sphingoid base length and number of hydroxyl groups in inositolphosphorylceramides were altered in sur7⌬, ydl222⌬, and yne194⌬ strains.The importance of membrane domains and local differences in membrane composition and structure has been described for several processes, including endocytosis, signaling through caveoli, and protein trafficking (17,32,34). For the yeast Saccharomyces cerevisiae, two reports have presented evidence for detergent-insoluble plasma membrane lipid rafts, which are important for protein sorting through the endoplasmic reticulum and Golgi (2, 30). Also, septins maintain the daughter cell plasma membrane as a domain with distinct markers from the mother's plasma membrane (4, 49). However, most yeast plasma membrane proteins, whether integral, peripheral, or glycosylphosphatidylinositol anchored, are dispersed evenly throughout the plasma membrane. With the notable exception of actin patch components, relatively few proteins are known to localize as patches or domains associated with the plasma membrane.SUR7 was originally identified as a multicopy suppressor of mutations in rvs161 and/or rvs167 (45). rvs161 and rvs167 mutants have reduced viability upon starvation (6, 13) and are also defective in actin polarization, bipolar bud site selection (6, 19, 44), endocytosis (35), and sporulation (12, 15). Overexpression of SUR7 suppressed rvs161 and rvs167 defects in actin polarization, bud site selection, and growth (45). rvs161 and rvs167 are also suppressed by loss-of-function mutations in the nonessential genes sur1, sur2, and sur4 (15). Sur1p, Sur2p, and Sur4p are a mannosyl-transferase, a hydroxylase, and an acyl chain elongation protein, respectively, all involved in sphingolipid biosynthesis (see references 16 and 42).The Sur7p family includes three members in S. cerevisiae: Sur7p, Ynl194p, and Ydl222p. They are predicted to be integral membrane proteins; each has a signal sequence and three transmembrane helices (45...

The lateral compartmentation of the yeast plasma membrane

“…The process is reversible, which suggests that the compartment as such is defined by other proteins that are not susceptible to the membrane potential changes. In addition to the H + -symporters above, four proteins of Sur7 family [18,36] and Nce102 with its close homologue Ygr131w [31] were localized in MCC. None of these proteins respond to changes in membrane potential [7,8] and thus, theoretically, they could play the role of those structure-defining elements of MCC.…”

Eisosome Ultrastructure and Evolution in Fungi, Microalgae, and Lichens

“…In both S. cerevisiae and S. pombe, eisosomes are actively excluded from cellular domains where new cell wall is being laid down, such as small buds, growing tips, and mitotic cleavage planes (9,14,43,78,79), suggesting that they may actually interfere with early fungal wall formation but participate in wall maturation.…”

“…These observations were brought together by Strádalová et al (5), who showed that the membrane compartment occupied by Can1p (MCC) localized to the invaginations seen with freeze fracture EM, and by Karotki et al (6), who showed that Pil1p localizes to the cytoplasmic surfaces of these invaginations. Additional proteins also associate with these punctate domains, in some cases in a transient fashion (17), and the MCC component is reportedly enriched in ergosterol (18) and influenced by phosphoinositide (6,19,20) and sphingolipid (14,17,(21)(22)(23)(24) levels. Hence, the current yeast model (25,26) proposes that Pil1p and Lsp1p, which contain membrane curvatureinducing BAR domains (6,27,28), together with Seg1p (29,30), form a submembrane complex (6) reportedly influenced by Pil1p phosphorylation (25); this complex then either creates or associates with the MCC domains, presumably inducing an inward curvature.…”

“…In 2002, Young et al (14) reported that green fluorescent protein (GFP)-tagged Sur7p, a tetraspanning plasma membrane protein of S. cerevisiae, localized to stable punctate domains of the yeast membrane; in 2004, a tagged proton-arginine transporter, Can1p, was found to colocalize with Sur7p (15); and in 2006, two tagged cytoplasmic proteins, Pil1p and Lsp1p, were localized to these domains (16). These observations were brought together by Strádalová et al (5), who showed that the membrane compartment occupied by Can1p (MCC) localized to the invaginations seen with freeze fracture EM, and by Karotki et al (6), who showed that Pil1p localizes to the cytoplasmic surfaces of these invaginations.…”

Microcompartments within the yeast plasma membrane

“…Accordingly, these patches were named MCC, for m embrane c ompartment of C an1, with an appearance of 50 -80 dots per cell and a diameter of approximately 300 nm. These microcompartments also house two other proton-dependent permeases for uracil and tryptophan, Fur4 and Tat2, respectively, as well as four proteins of the Sur7 family (Young et al , 2002 ;Malinska et al , 2004 ). The MCCs are associated with the eisosomes (discussed below) at the inner leaflet of ergosterol-enriched regions of the plasma membrane, which invaginate in furrow-like structures (Grossmann et al , 2008 ;Stradalova et al , 2009 ).…”