THE mad2-l mutation inactivates the cell-cycle feedback control that prevents budding yeast cells from leaving mitosis until spindle assembly is complete1. The gene product of MAD2 shows significant sequence similarity to the α-subunit of prenyltransferases2. Here we isolate a new temperature-sensitive mad2 mutant, mad2-2ts, and find that Mad2p is required for the membrane association of Yptlp and Sec4p, two prenylated small GTP-binding proteins involved in protein trafficking. Extracts from mad2-2ts mutant cells fail to geranylgeranylate a number of substrates at the non-permissive temperature. mad2-2ts is synthetically lethal with bet2-1, a mutation in the gene that encodes for the β-subunit of the Yptlp and Sec4p geranylgeranyl transferase3,4. Therefore MAD2 and BET2 gene products may physically interact to form a geranylgeranyl transferase complex. In addition, the difference between the phenotypes of mad2-l and mad2-2ts suggests that MAD2 has distinct roles in protein transport and the mitotic feedback control.
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