SUN Family Proteins Sun4p, Uth1p and Sim1p Are Secreted from Saccharomyces cerevisiae and Produced Dependently on Oxygen Level

Abstract

The SUN family is comprised of proteins that are conserved among various yeasts and fungi, but that are absent in mammals and plants. Although the function(s) of these proteins are mostly unknown, they have been linked to various, often unrelated cellular processes such as those connected to mitochondrial and cell wall functions. Here we show that three of the four Saccharomyces cerevisiae SUN family proteins, Uth1p, Sim1p and Sun4p, are efficiently secreted out of the cells in different growth phases and their production is affected by the level of oxygen. The Uth1p, Sim1p, Sun4p and Nca3p are mostly synthesized during the growth phase of both yeast liquid cultures and colonies. Culture transition to slow-growing or stationary phases is linked with a decreased cellular concentration of Sim1p and Sun4p and with their efficient release from the cells. In contrast, Uth1p is released mainly from growing cells. The synthesis of Uth1p and Sim1p, but not of Sun4p, is repressed by anoxia. All four proteins confer cell sensitivity to zymolyase. In addition, Uth1p affects cell sensitivity to compounds influencing cell wall composition and integrity (such as Calcofluor white and Congo red) differently when growing on fermentative versus respiratory carbon sources. In contrast, Uth1p is essential for cell resistance to boric acids irrespective of carbon source. In summary, our novel findings support the hypothesis that SUN family proteins are involved in the remodeling of the yeast cell wall during the various phases of yeast culture development and under various environmental conditions. The finding that Uth1p is involved in cell sensitivity to boric acid, i.e. to a compound that is commonly used as an important antifungal in mycoses, opens up new possibilities of investigating the mechanisms of boric acid’s action.

BcSUN1, a B. cinerea SUN-Family Protein, Is Involved in Virulence

“…On the other hand, as discussed above, BcSUN1 plays a role in the cell wall. Its S. cerevisiae homologues SUN4, UTH1 and SIM1 have been reported to have having multiple cellular locations, either in the cell wall, the extracellular space, or in the mitochondria (Kuznetsov et al, 2013). …”

“…β-Glucosidase SUN family members have been extensively studied in yeast and have been linked to diverse cellular functions such as cell wall biogenesis and septation, mitochondrial biogenesis, aging and DNA replication (Camougrand et al, 2000; Mouassite et al, 2000a; Velours et al, 2002; Hiller et al, 2007; Kuznetsov et al, 2013). BcSUN1 is a glycoprotein secreted by B. cinerea (Espino et al, 2010; González et al, 2014) that belongs to the Group-I of the SUN family, showing the highly conserved C -terminal region characteristic for this group of proteins.…”

“…Similarly to UTH1, SUN4 has also been found both in the cell wall and in mitochondria (Cappellaro et al, 1998; Mouassite et al, 2000a). Recently, UTH1, SIM1 and SUN4 have also been described as secreted proteins, and their production was affected by the level of oxygen (Kuznetsov et al, 2013). In C. albicans , on the contrary, only two members of the SUN family have been identified, SUN41 and SUN42, both of which are involved in remodeling the cell wall and are essential for cell separation (Firon et al, 2007; Hiller et al, 2011).…”

Cellular localization of Sun4p and its interaction with proteins in the yeast birth scar

“…High Sun4p levels identified by WB in cell wall extracts of the BY-dse2-Sun4p-HA strain (with protein levels similar to wt) in parallel with the absence of Sun4p-HA in the birth scars (as observed by IF) indicate that Sun4p is also present in the cell wall, with its C-and N-termini facing inside the cells, which is the direction opposite to that observed in the birth scar. Cell wall localization of Sun4p is also supported by the fact that the sun4D strain significantly decreases sensitivity to zymolyase, 2 implicating Sun4p role in cell wall remodeling. The dual localization of Sun4p may signify a dual function, with one related to birth scar localization in the separation of daughter cells from mother cells (together with other glucanases probably involved in secondary septum destruction) 14 and the other function linked to the cell wall localization of proteins in cell wall organization during bud growth.…”

“…1 This protein is also efficiently released from cells growing either in liquid medium or within yeast colonies. 2 Although little is known about the function of Sun4p and other SUN family proteins, Sun4p may be involved in cell separation, which is delayed in the sun4D strain; this strain often possesses more than one bud, and daughter cells often remain attached to the mother cell. 3 Sun4p homolog from Candida albicans, Sun41p, plays a role in virulence and biofilm formation, cell attachment to a substrate, and cell separation and hyphae formation, leading to the hypothesis that the SUN family proteins in C. albicans are involved in cell integrity maintenance during cell division, accompanied by cell wall remodeling.…”