Functional analysis of "MEMO / MHO1", an evolutionary conserved gene, in yeast and mammalian cells

The protein Memo (Mediator of ErbB2 driven cell motility) was identified in a screen for ErbB2 receptor tyrosine kinase (RTK) interacting proteins that have roles in cancer cell motility. A single Memo protein of 297 amino acids is encoded in the human genome. Memo is evolutionarily conserved and homologs are found in all branches of life. The human and the yeast protein share an identity of more than 40% and a similarity of more than 50%. Memo is not homologous to any known signaling proteins and based on its conservation we expect it to have functions in addition to promoting motility in response to RTK activation. In the work described here, we used the model organism S. cerevisiae to characterize Mho1 (Yjr008wp) and to investigate its function in yeast. MHO1 expression is strongly induced in conditions of stress. In stationary phase, one stress condition, a high percentage of Memo is present in the nucleus. In mammalian cells, Memo is also found throughout the cell. Memo has no obvious NLS (nuclear localization sequence), however, an NES (nuclear export sequence) is present in all Memo homologs. In mammalian cells, blocking nuclear export with Leptomycin B led to nuclear Memo accumulation, suggesting that it is actively exported from the nucleus. Since invasive growth in S. cerevisiea can be induced by stress, e.g., nitrogen deprivation, or alcohol induced, we tested the role of Mho1 in this response. Deletion of MHO1 had no effect on the formation of pseudohyphoa or invasion. Growth of mho1Δ cells was not affected by stress inducers including (HU, CoCl2, Heat-shock, Latrunculin, Nocodazol). Interestingly, however, overexpression of Mho1 blocked the ability of the yeast cells to invade. In a synthetic lethal (SL) screen we found MHO1 as a novel SL partner of PLC1. Plc1 is the only phospholipase C in yeast and hydrolyzes phosphatidylinositol 4,5-biphosphate (PIP2) to generate the signaling molecules inositol 1,4,5-triphosphate (IP3) and 1,2-diacylglycerol (DAG). In the absence of MHO1 and PLC1, double deleted spores still germinate but proliferation is impaired after 2 to 10 cell cycles by an unknown mechanism. Introduction of human MEMO into the memoΔplc1Δ strain could rescue the SL phenotype showing that the specific function of Mho1/Memo needed to overcome the synthetic lethal phenotype is conserved.