EXPRESSION OF BAK IN S-POMBE RESULTS IN A LETHALITY MEDIATED THROUGH INTERACTION WITH THE CALNEXIN HOMOLOG CNX1

Expression studies in the yeast S. pombe have been utilised to establi sh the basis for a genetic analysis designed to identify the lethal pa rtners of the pro-apoptotic proteins bak and bar. Bak expression in S. pombe is lethal and this lethality is rescued by co-expression of bcl -2 or bcl-x(L). S. pombe cells expressing bak have a terminal phenotyp e in which the majority of cells are blocked in the G1 phase of the ce ll cycle while the remainder of cells, unable to complete M-phase, mis -coordinate the timing of subsequent events in the cell cycle, Althoug h bar expression in S. pombe gives rise to a slow growth phenotype, no t a lethality, bar expressing cells display the same cell cycle phenot ypes described for bak, Electron microscopy of cells expressing bak re veals a dramatic accumulation of large vesicular structures, A two-hyb rid screen designed to identify S. pombe proteins which interact with bak, isolated the S. pombe calnexin homologue cnx1. Genetic analysis d emonstrates that the Cnx1 domain which binds to bak in two-hybrid expe riments, is necessary for bak lethality in S. pombe. This report ident ifies a lethal interacting partner for bak and the observations sugges t a model for bak mediated lethality which can be tested in higher cel ls.