It has been proposed that beta-1,6-glucan is synthesized intracellularly
in a stepwise process involving so-called KRE genes located in
the various compartments of the secretory pathway. Here we propose that
beta-1,6-glucan synthesis takes place at the cell surface. First, the
proposed glucosyl transferases Kre6 and Skn1 are required for full
N -glycosylation of the vacuolar protein carboxypeptidase Y, the
plasma membrane-bound protein Gas1, the periplasmic enzyme invertase,
and the cell wall protein alpha-agglutinin. This excludes that Kre6 and
Skn1 are directly involved in beta-1,6-glucan synthesis. Second, using
immunoelectron microscopy we find beta-1,6-glucan only at the cell
surface, but never intracellularly. Third, plasma membrane-derived
vesicles isolated by gel filtration contain beta-1,6-glucan in contrast
to post-Golgi secretory vesicles. These data are consistent with a model
in which KRE genes are involved in the synthesis of an acceptor
structure for later beta-1,6-glucan addition and in which beta-1,6-glucan is synthesized at the plasma membrane.
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