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Vol. 11, Issue 5, 1611-1630, May 2000
Department of Biochemistry, University of Illinois at
Urbana-Champaign, Urbana, Illinois 61801
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 the
gpi11 deletion, but the rescued haploids are temperature
sensitive. Abolition of Gpi11p or Pig-Fp function in
GPI11 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.
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