The role of Mon2p in membrane trafficking, protein sorting, and cell growth

Jem A. Efe, Scott D. Emr. Division of Biology & HHMI, UC San Diego, 9500 Gilman Drive, La Jolla, CA, 92093-0668, USA.

MON2 was first identified in a genetic screen for mutants that exhibit synthetic lethality with deletion of YPT51. Accordingly, a strain lacking MON2 exhibits defects in protein sorting and vacuole biogenesis, as well as impaired growth and endocytosis (Singer-Krueger and Ferro-Novick 1997). Furthermore, Mon2p-GFP localizes to punctate structures in the cytosol and is peripherally associated with light membranes (Jochum, Jackson et al. 2002). Our studies have shown that mon2delta cells, in addition to a delay in ALP processing, exhibit a partial block in APeI maturation via the constitutive Cvt pathway. This defect resembles that of the GARP complex (VPS51-54) mutants as well as ypt6delta. Electron microscopy of the mon2delta strain has revealed an accumulation of membranous inclusions in the cytoplasm. These findings suggest a role for Mon2p at the late Golgi/early endosomes, an observation supported by colocalization of Mon2-GFP with Sec7-DsRed and DsRed-PH chimeras. We have found that deletion of MON2 is synthetically lethal with a set of genes critical for retrograde traffic to the late Golgi. Moreover, proteins known to recycle through the Golgi are mislocalized in the mon2delta strain at high temperature. Based on structure-function studies, we have concluded that the N terminus of the protein mediates its localization, while the C terminus harbors a critical functional domain. Our recent findings on the localization and activity of Mon2p will be presented.

Program Nr. 227B from 2004 Yeast meeting

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