Little is known about how proteins are retained in the Golgi complex. For the budding yeast protein Mnn1p, either the transmembrane domain or the lumenal domain is sufficient for retention. On page , Reynolds et al. report that the MAP kinase Hog1p is needed for retention mediated by the lumenal domain. Hog1p is part of a pathway for sensing hyperosmotic stress, and Reynolds et al. find that other components of this pathway are necessary for localization of the Mnn1p lumenal domain.
Without Hog1p, the lumenal domain is secreted, whereas the full-length protein slips from earlier Golgi compartments to the trans-Golgi network. Whether this affects the α-1,3-mannosyltransferase function of Mnn1p is not known. It is certainly possible, since the lumen of later Golgi compartments may differ in acidity, divalent cation concentrations, or the availability of sugar nucleotide donors.
A change in Mnn1p activity could alter the cell wall, which...
