A genetic screen for increasing metabolic flux in the isoprenoid pathway of Saccharomyces cerevisiae: Isolation of SPT15 mutants using the screen

A genetic screen to identify mutants that can increase flux in the isoprenoid pathway of yeast has been lacking. We describe a carotenoid-based visual screen built with the core carotenogenic enzymes from the red yeast Rhodosporidium toruloides. Enzymes from this yeast displayed the required, higher capacity in the carotenoid pathway. The development also included the identification of the metabolic bottlenecks, primarily phytoene dehydrogenase, that was subjected to a directed evolution strategy to yield more active mutants. To further limit phytoene pools, a less efficient version of GGPP synthase was employed. The screen was validated with a known flux increasing gene, tHMG1. New mutants in the TATA binding protein SPT15 were isolated using this screen that increased the yield of carotenoids, and an alternate isoprenoid, α-Farnesene confirming increase in overall flux. The findings indicate the presence of previously unknown links to the isoprenoid pathway that can be uncovered using this screen.

Accelerated proteasomal degradation of membrane Ig heavy chains

Membrane IgG H chains turn over considerably more rapidly than secretory Ig H chains in the 18-81 A2 pre-B cell line. This rapid degradation occurs in proteasomes. N-Glycosylated membrane Ig H chains accumulate in the endoplasmic reticulum in the presence of proteasomal inhibitors, suggesting that retrotranslocation and proteasomal degradation of membrane Ig H chains may be closely coupled processes. Accelerated proteasomal degradation of membrane Ig H chains was also observed in transfected nonlymphoid cells. At steady state, the membrane form of the H chain associates more readily with Bip and calnexin than its secretory counterpart. The preferential recognition of membrane, as opposed to secretory, Ig H chains by some endoplasmic reticulum chaperones, may provide an explanation for the accelerated proteasomal degradation of the former.