We studied the phosphorylation of the inner core region of N-linked oligosaccharides in the mannan defective mutant Saccharomyces cerevisiae mnn2 which was described as unable to synthesize branches on the outer chain. We performed structural studies of the N-oligosaccharides synthesized by the strains mnn2, mnn1mnn2mnn9 and mnn1mnn9ldb8, and the results are compared with previously published structural data of mnn1mnn2mnn10 and mnn1mnn9 [Hernández, L.M., Ballou, L., Alvarado, E., Tsai, P.-K. and Ballou, C.E. (1989) J. Biol. Chem. 264, 13648–13659]. We conclude that the mnn2/ldb8 mutation is responsible for the inhibition of incorporation of phosphate to mannose A3 (see below), a particular phosphorylation site of the inner core, while phosphorylation at the other possible site (mannose C1) is allowed, although it is also reduced. *Phosphorylation sites in mnn1mnn9.
