Cell Biology

Molecular Bulldozing

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Science  22 Sep 2000:
Vol. 289, Issue 5487, pp. 2005-2007
DOI: 10.1126/science.289.5487.2005e

In order to control a variety of gene expression processes the cell stores control factors, known as transcription factors, away from nuclear genes in the cytosol. To be doubly sure, the cell sometimes even stores these proteins as integral membrane proteins; in response to a specific signal, the cell then must liberate the transcription factors from the appropriate membrane so that they can enter the nucleus and activate their target genes.

Hoppe et al. examined the details of a system that regulates the ratio of saturated and unsaturated fatty acid synthesis, and discovered an interesting twist. In the case of two related yeast transcription factors known as SPT23 and MGA2, the cell uses the cytosolic ubiquitin-proteasome degradative pathway to cleave the transcription factor from the membrane. The proteasome pathway is not usually used in site-specific cleavage, but generally degrades its substrates to small peptides. The details of how the proteasome makes its way to the requisite cleavage site near the membrane and of how complete degradation is blocked remain to be elucidated, but as ever in biology, it appears that there are as many variations on a theme as there are themes to discover. — SMH

Cell102, 577 (2000).

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