Glycosylphosphatidylinositols (GPIs) are critical for membrane anchoring and intracellular transport of certain secretory proteins. GPIs have a conserved trimannosyl core bearing a phosphoethanolamine (EthN-P) moiety on the third mannose (Man-3) through which the glycolipid is linked to protein, but diverse GPI precursors with EthN-Ps on Man-1 and Man-2 have also been described. We report on two essential yeast genes whose products are required late in GPI assembly. GPI11 (YDR302w) encodes a homologue of human Pig-Fp, a protein implicated in the addition of EthN-P to Man-3. PIG-F complements thegpi11 deletion, but the rescued haploids are temperature sensitive. Abolition of Gpi11p or Pig-Fp function inGPI11 disruptants blocks GPI anchoring and formation of complete GPI precursors and leads to accumulation of two GPIs whose glycan head groups contain four mannoses but differ in the positioning and number of side chains, probably EthN-Ps. The less polar GPI bears EthN-P on Man-2, whereas the more polar lipid has EthN-P on Man-3. The latter finding indicates that Gpi11p is not required for adding EthN-P to Man-3. Gpi13p (YLL031cp), a member of a family of phosphoryltransferases, is a candidate for the enzyme responsible for adding EthN-P to Man-3. Depletion of Gpi13p in a Gpi11p-defective strain prevents formation of the GPI bearing EthN-P on Man-3, and Gpi13p-deficient strains accumulate a Man4-GPI isoform that bears EthN-P on Man-1. We further show that the lipid accumulation phenotype of Gpi11p-deficient cells resembles that of cells lacking Gpi7p, a sequence homologue of Gpi13p known to add EthN-P to Man-2 of a late-stage GPI precursor. This result suggests that in yeast a Gpi11p-deficiency can affect EthN-P addition to Man-2 by Gpi7p, in contrast to the Pig-Fp defect in mammalian cells, which prevents EthN-P addition to Man-3. Because Gpi11p and Pig-Fp affect EthN-P transfer to Man-2 and Man-3, respectively, these proteins may act in partnership with the GPI-EthN-P transferases, although their involvement in a given EthN-P transfer reaction varies between species. Possible roles for Gpi11p in the supply of the EthN-P donor are discussed. Because Gpi11p- and Gpi13p-deficient cells accumulate isoforms of Man4-GPIs with EthN-P on Man-2 and on Man-1, respectively, and because the GPIs that accumulate in Gpi11p-defective strains are likely to have been generated independently of one another, we propose that the yeast GPI assembly pathway is branched.

The major glycosylphosphatidylinositols (GPIs) transferred to protein in mammals and trypanosomes contain three mannoses. In Saccharomyces cerevisiae, however, the GPI transferred to protein bears a fourth, ␣1,2-linked Man on the ␣1,2-Man that receives the phosphoethanolamine (EthN-P) moiety through which GPIs become linked to protein. We report that temperaturesensitive smp3 mutants accumulate a GPI containing three mannoses and that smp3 is epistatic to the gpi11, gpi13, and gaa1 mutations, which normally result in the accumulation of Man 4 -GPIs, including the presumed substrate for the yeast GPI transamidase. The Smp3 protein, which is encoded by an essential gene, is therefore required for addition of the fourth Man to yeast GPI precursors. The finding that smp3 prevents the formation of the Man 4 -GPI that accumulates when addition of EthN-P to Man-3 is blocked in a gpi13 mutant suggests that the presence of the fourth Man is important for transfer of EthN-P to Man-3 of yeast GPIs. The Man 3 -GPI that accumulates in smp3 is a mixture of two dominant isoforms, one bearing a single EthN-P side branch on Man-1, the other with EthN-P on Man-2, and these isoforms can be placed in separate arms of a branched GPI assembly pathway. Smp3-related proteins are encoded in the genomes of Schizosaccharomyces pombe, Candida albicans, Drosophila melanogaster, and Homo sapiens and form a subgroup of a family of proteins, the other groups of which are defined by the Pig-B(Gpi10) protein, which adds the third GPI mannose, and by the Alg9 and Alg12 proteins, which act in the dolichol pathway for N-glycosylation. Because Man 4 -containing GPI precursors are normally formed in yeast and Plasmodium falciparum, whereas addition of a fourth Man during assembly of mammalian GPIs is rare and not required for GPI transfer to protein, Smp3p-dependent addition of a fourth Man represents a target for antifungal and antimalarial drugs.

Yeast and human glycosylphosphatidylinositol (GPI) precursors differ in the extent to which a fourth mannose is present as a side branch of the third core mannose. A fourth mannose addition to GPIs has scarcely been detected in studies of mammalian GPI synthesis but is an essential step in the Saccharomyces cerevisiae pathway. We report that human SMP3 encodes a functional homolog of the yeast Smp3 GPI fourth mannosyltransferase. Expression of hSMP3 in yeast complements growth and biochemical defects of smp3 mutants and permits in vivo mannosylation of trimannosyl (Man 3 )-GPIs. Immunolocalization shows that hSmp3p resides in the endoplasmic reticulum in human cells. Northern analysis of mRNA from human tissues and cell lines indicates that hSMP3 is expressed in most tissues, with the highest levels in brain and colon, but its mRNA is nearly absent from cultured human cell lines. Correspondingly, increasing expression of hSMP3 in cultured HeLa cells causes abundant formation of three putative tetramannosyl (Man 4 )-GPIs. Our data indicate that hSmp3p functions as a mannosyltransferase that adds a fourth mannose to certain Man 3 -GPIs during biosynthesis of the human GPI precursor, and suggest it may do so in a tissue-specific manner.Glycosylphosphatidylinositols (GPIs) 1 are essential glycolipids synthesized by all eukaryotes. Many GPIs become covalently attached to the carboxyl termini of various secretory proteins and serve to anchor them to the exterior face of the plasma membrane (1, 2). Others remain protein-free and are distributed in the membranes of major cellular organelles and the plasma membrane (3, 4). GPIs are synthesized in the endoplasmic reticulum (ER) by stepwise addition of components to phosphatidylinositol. The end product of GPI synthesis is a "complete precursor" (5) that is substrate for the GPI transamidase complex that attaches it to proteins. Many of the steps and enzymes of GPI precursor assembly are conserved between humans and Saccharomyces cerevisiae and produce precursors with a common core structure of EthN-PO 4 -6Man␣1,2Man␣1,6Man␣1,4GlcN␣1,6Ins-PO 4 -lipid. In both pathways, the glycan portion of the GPI may be modified further with side branching ethanolamine phosphate (EthN-P) moieties on the first and second mannoses (6 -19).A notable difference in GPI structure between yeast and mammals is the extent to which a fourth mannose (Man-4) is present as a ␣1,2-linked side branch of the third mannose (Man-3). In S. cerevisiae, late stage intermediates in GPI precursor synthesis (17, 19 -21), the presumed GPI transamidase substrates (5), and protein-bound GPIs (22) all contain four mannoses. Addition of Man-4 to trimannosyl-GPIs (Man 3 -GPIs) by the essential Smp3 mannosyltransferase is a mandatory step in yeast GPI precursor assembly which precedes the addition of the terminal EthN-P to Man-3 through which the GPI is ultimately attached to protein (23). Thus, it is probable that all yeast GPI transamidase substrates bear four mannoses. Conversely, studies of the synthesis of mammal...

Yeast mcd4-174 mutants are blocked in glycosylphosphatidylinositol (GPI) anchoring of protein, but the stage at which GPI biosynthesis is interrupted in vivo has not been identified, and Mcd4p has also been implicated in phosphatidylserine and ATP transport. We report that the major GPI that accumulates in mcd4-174 in vivo is Man(2)-GlcN-(acyl-Ins)PI, consistent with proposals that Mcd4p adds phosphoethanolamine to the first mannose of yeast GPI precursors. Mcd4p-dependent modification of GPIs can partially be bypassed in the mcd4-174/gpi11 double mutant and in mcd4Delta; mutants by high-level expression of PIG-B and GPI10, which respectively encode the human and yeast mannosyltransferases that add the third mannose of the GPI precursor. Rescue of mcd4Delta; by GPI10 indicates that Mcd4p-dependent addition of EthN-P to the first mannose of GPIs is not obligatory for transfer of the third mannose by Gpi10p.