The plasma membrane of the yeast
Saccharomyces cerevisiae contains stably distributed lateral domains of specific composition and structure, termed MCC (
membrane
compartment of arginine permease
Can1). Accumulation of Can1 and other specific proton symporters within MCC is known to regulate the turnover of these transporters and is controlled by the presence of another MCC protein, Nce102. We show that in an
NCE102 deletion strain the function of Nce102 in directing the specific permeases into MCC can be complemented by overexpression of the
NCE102 close homolog
FHN1 (the previously uncharacterized
YGR131W) as well as by distant
Schizosaccharomyces pombe homolog
fhn1 (
SPBC1685.13). We conclude that this mechanism of plasma membrane organization is conserved through the phylum
Ascomycota. We used a hemagglutinin (HA)/Suc2/His4C reporter to determine the membrane topology of Nce102. In contrast to predictions, its N and C termini are oriented toward the cytosol. Deletion of the C terminus or even of its last 6 amino acids does not disturb protein trafficking, but it seriously affects the formation of MCC. We show that the C-terminal part of the Nce102 protein is necessary for localization of both Nce102 itself and Can1 to MCC and also for the formation of furrow-like membrane invaginations, the characteristic ultrastructural feature of MCC domains.Stable lateral domains coexist within the plasma membrane of the yeast
Saccharomyces cerevisiae. Nce102, a protein originally thought to be involved in nonclassical export (
6) and more recently in sensing sphingolipids (
10), is the main organizer of one type of these domains, termed MCC (
membrane
compartment of
Can1) (
25). MCC consists of evenly distributed, isolated patches enriched in sterols and specific proteins (
15,
16,
25,
26). We showed that MCC-specific proton symporters accumulate in these patches in a reversible, membrane potential-dependent manner. This Nce102-mediated transient MCC accumulation plays a key role in the turnover of the transporters (
16). Each MCC patch is accompanied by an eisosome, a cytosolic complex located directly beneath the membrane (
36).In an early freeze-etching study, Moor and Mühlethaler (
28) demonstrated that the yeast plasma membrane contains numerous furrow-like invaginations. Recently, MCC patches were identified with these plasma membrane structures, and Nce102 was shown to be necessary for furrow formation. On the ultrastructural level, the MCC patches of
nce102Δ cells appeared as flat, smooth, elongated areas within an otherwise particle-rich plasma membrane (
32).There is now increasing evidence that cytosolic Pil1, a primary component of eisosomes, is a prerequisite for MCC patch formation. It marks the sites where Nce102 and the MCC-specific transporters will subsequently accumulate (
16,
23,
29). Data published so far do not indicate a direct involvement of cytoskeletal components in this process (
26). Accordingly, markers of classical endocytosis, which are coupled to the cortical patches of actin, were localized outside the MCC (
16).In this paper we examine the contribution of Nce102 to the organization of MCC patches and of furrow-like invaginations. Our results indicate that, in contrast to the prediction of four transmembrane domains (TMDs), the Nce102 molecule might span the plasma membrane only twice, the C and N termini being oriented toward the cytoplasm. We find that the C-terminal 6 amino acids of Nce102 are essential for MCC patch formation as well as for the formation of the furrow-like membrane invaginations. In addition it is shown that this Nce102 function is phylogenetically conserved among
Ascomycota.
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