Figure 8. Efr3 recruits PIK patch components to the PM. (A) Growth assay of EFR3 wild type and efr3-1 temperature-sensitive yeast at 26 and 38°C. (B) Phosphoinositide levels of EFR3 and efr3-1 at 38°C. Data are presented as means and SD (error bars) of two independent experiments. (C) Cellular localization of the PtdIns4P reporter GFP-2xPHOsh2 in efr3-1 yeast at the permissive (top) and restrictive (bottom) temperatures. Bars, 4 μm. (D) The localization proﬁle of teto-EFR3 GFP-Stt4 in the absence (top) or presence (bottom) of doxycycline and the corresponding DIC image. Arrows indicate PIK patch localization. Bars, 4 μm. (E) The locali- zation proﬁle of teto-YPP1 Efr3-GFP in the ab- sence (top) or presence (bottom) of doxycycline and the corresponding DIC images. Arrows indicate PIK patch localization. Bars, 4 μm. Ypp1 deﬁnes a well conserved protein Ypp1 (YGR198w) is a TPR domain–containing protein with homo- logues in Drosophila, Mus musculus, and Homo sapiens. TPR do- mains are a repeating unit of two antiparallel helices separated by a turn that form a structure resembling a spiraling staircase (Scheufler et al., 2000). These domains are a common structural unit throughout evolution from bacteria to humans and have been shown to be important for directing specific protein–protein inter- actions (Blatch and Lassle, 1999). The most conspicuous TPR domain–containing proteins are cochaperones, such as Sti1, non- client binding partners that participate in chaperone-mediated folding of proteins (Prodromou et al., 1999). Ypp1’s precise role as a TPR domain–containing protein is consistent with what has al- ready been established for proteins containing this structural fea- ture. As we have demonstrated in this paper Ypp1 is essential for maintaining the integrity of the large PIK patch complex. In the absence of Ypp1, Stt4 is unstable, mislocalizes to the cytoplasm, and is quickly degraded. Therefore, one important role for Ypp1 may be to assist the folding of Stt4 and/or structurally maintain the integrity of the PtdIns 4-kinase within the PIK patch complex. We were unable to identify a role for Ypp1 independent of Stt4 function, but it is exciting to speculate that the essential Ypp1–Stt4 interaction is maintained throughout evolution. For example, homologues of Ypp1 in higher eukaryotes may also be important for maintaining the structural integrity of protein complexes. For example, YPP1 has a potential mouse homo- logue, TTC7a, which previous work has found to be essential for normal cell functions. That study determined that a sponta- neous retroviral insertion into a TPR domain of TTC7a produced a flaky skin (fsn) mutation in mice (Helms et al., 2005). The ab- normality causes symptoms similar to lupus such as anemia and production of anti-dsDNA autoantibodies. Interestingly, the analo- gous TPR deletion in Ypp1 yielded a nonfunctional protein unable to bind Stt4 or rescue ypp1 cells (unpublished data). Helms et al. (2005) speculated that an interaction with a critical TTC7a binding partner is disrupted in fsn mice. Could the Stt4 mouse homologue, PtdIns(4)KIII , be that protein? Are some mammalian PIK patches no longer stable in the TTC7a mouse mutant? Future work in the mouse will be needed to directly ad- dress this issue. 1070 JCB • VOLUME 183 • NUMBER 6 • 2008