Cell Biology

Two Roads Diverged

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Science  25 May 2001:
Vol. 292, Issue 5521, pp. 1451
DOI: 10.1126/science.292.5521.1451d

The posttranslational modification of proteins by the attachment of the protein ubiquitin is known to play a role in the regulated degradation of many proteins by the proteasome. A role for ubiquitination in triggering the internalization of membrane receptors in yeast also is well established.

Now, Helliwell et al. define another function for ubiquitination in controlling the intracellular roadmap followed by the general amino acid permease (Gap1p) in yeast. When sources of nutritional nitrogen are limiting, Gap1p travels to the cell surface; in times of plenty, Gap1p is routed to the vacuole for degradation. In cells that expressed high levels of two proteins, Bul1p and Bul2p, which promote the attachment of ubiquitin, Gap1p was transported to the vacuole irrespective of nitrogen levels. Conversely, when the bul1 and bul2 genes were deleted, the Gap1 protein was delivered predominantly to the plasma membrane. Ubiquitinated forms of Gap1p were abundant in the former situation and greatly diminished in the deletion strains. Furthermore, truncation of Gap1p, which forestalled ubiquitination, enhanced delivery to the cell surface. Thus, it appears that ubiquitin can direct the paths taken upon exit from the Golgi complex, as well as during endocytosis from the cell surface. — SMH

J. Cell Biol.153, 649 (2001).

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