Roles of an N-terminal coiled-coil-containing domain in the localization and function of Bem3, a Rho GTPase-activating protein in budding yeast

Crystal Structure of the Epo1-Bem3 Complex for Bud Growth

Tubules of the endoplasmic reticulum (ER) spread into the buds of yeast by an actin-based mechanism and, upon entry, become attached to the polarisome, a proteinaceous micro-compartment below the tip of the bud. The minimal tether between polarisome and cortical ER is formed by a protein complex consisting of Epo1, a member of the polarisome, Scs2, a membrane protein of the ER and Cdc42 guanosine triphosphatase-activating protein Bem3. Here, we report the crystal structure of a complex between Epo1 and Bem3. In addition, we characterize through the hydrogen/deuterium (H/D) exchange assay the interface between Scs2 and Epo1. Our findings provide a first structural insight into the molecular architecture of the link between cortical ER and the polarisome.

The Sur7/PalI family transmembrane protein Tos7 (Yol019w) plays a role in secretion in budding yeast

Tos7 (Yol019w) is a Sur7/PalI family transmembrane protein in the budding yeast Saccharomyces cerevisiae. Since the deletion of TOS7 did not affect growth or cell morphology, the cellular roles of Tos7 have not been established previously. Here, we show that high-copy TOS7 expression suppressed the growth defect of the secretion-defective RGA1-C term-overexpressing mutant and sec15-1 mutant. Moreover, Tos7 physically interacted with Boi2 and the Rho GTPase Rho3, two key regulators of exocyst assembly, suggesting that Tos7 plays a role in secretion. We also show that the deletion of TOS7 rendered the cells more sensitive to the cell wall-disrupting agents Congo red and calcofluor white while high-copy TOS7 expression had an opposite effect, suggesting that Tos7 affects cell wall organization. Finally, we show that Tos7 localized to punctate patches on the plasma membrane that were largely co-localized with the plasma membrane microdomains named MCC (membrane compartment of Can1). Together, these results suggest that Tos7 contributes to cell surface-related functions. Tos7 is likely an auxiliary component of MCC/eisosome that specifically interacts with the secretory pathway.

Roles of the PH, coiled-coil and SAM domains of the yeast polarity protein Boi2 in polarity-site localization and function in polarized growth

Boi1 and Boi2 are paralogous proteins essential for bud formation in budding yeast. So far, the domains that target Boi1/Boi2 to the polarity sites and function in bud formation are not well understood. Here, we report that a coiled-coil domain of Boi2 cooperates with the adjacent PH domain to confer Boi2's bud-cortex localization and major function in cell growth. The PH domain portion of the PH-CC bi-domain interacts with the Rho GTPases Cdc42 and Rho3 and both interactions are independent of the GTP/GDP-bound state of each GTPase. Interestingly, high-copy RHO3 and BOI2 but not CDC42 suppressed the growth defect of RGA1-C538 overexpression and the sec15-1 mutant and this BOI2 function depends on RHO3, suggesting that Boi2 may function in the Rho3 pathway. The SAM domain of Boi2 plays an essential role in high-copy suppression of the two mutants as well as in the early bud-neck localization of Boi2. The SAM domain and the CC domain also interact homotypically. They are likely involved in the formation of Boi2-containing protein complex. Our results provide new insights in the localization and function of Boi2 and highlight the importance of the PH-CC bi-domain and the SAM domain in Boi2's localization and function.

H2B ubiquitination: Conserved molecular mechanism, diverse physiologic functions of the E3 ligase during meiosis

RNF20/Bre1 mediated H2B ubiquitination (H2Bub) has various physiologic functions. Recently, we found that H2Bub participates in meiotic recombination by promoting chromatin relaxation during meiosis. We then analyzed the phylogenetic relationships among the E3 ligase for H2Bub, its E2 Rad6 and their partner WW domain-containing adaptor with a coiled-coil (WAC) or Lge1, and found that the molecular mechanism underlying H2Bub is evolutionarily conserved from yeast to mammals. However, RNF20 has diverse physiologic functions in different organisms, which might be caused by the evolutionary divergency of their domain/motif architectures. In the current extra view, we not only elucidate the evolutionarily conserved molecular mechanism underlying H2Bub, but also discuss the diverse physiologic functions of RNF20 during meiosis.