Uneven distribution and local concentration of protein complexes on distinct membrane cortices is a fundamental property in numerous biological processes, including Drosophila neuroblast (NB) asymmetric cell divisions and cell polarity in general. In NBs, the cell fate determinant Numb forms a basal crescent together with Pon and is segregated into the basal daughter cell to initiate its differentiation. Here we discover that Numb PTB domain, using two distinct binding surfaces, recognizes repeating motifs within Pon in a previously unrecognized mode. The multivalent Numb-Pon interaction leads to high binding specificity and liquid-liquid phase separation of the complex. Perturbations of the Numb/Pon complex phase transition impair the basal localization of Numb and its subsequent suppression of Notch signaling during NB asymmetric divisions. Such phase-transition-mediated protein condensations on distinct membrane cortices may be a general mechanism for various cell polarity regulatory complexes.

The highly conserved multifunctional polymerase-associated factor 1 (Paf1) complex (PAF1C), composed of five core subunits Paf1, Leo1, Ctr9, Cdc73, and Rtf1, participates in all stages of transcription and is required for the Rad6/Bre1-mediated monoubiquitination of histone H2B (H2Bub). However, the molecular mechanisms underlying the contributions of the PAF1C subunits to H2Bub are not fully understood. Here, we report that Ctr9, acting as a hub, interacts with the carboxyl-terminal acidic tail of Rad6, which is required for PAF1C-induced stimulation of H2Bub. Importantly, we found that the Ras-like domain of Cdc73 has the potential to accelerate ubiquitin discharge from Rad6 and thus facilitates H2Bub, a process that might be conserved from yeast to humans. Moreover, we found that Rtf1 HMD stimulates H2Bub, probably through accelerating ubiquitin discharge from Rad6 alone or in cooperation with Cdc73 and Bre1, and that the Paf1/Leo1 heterodimer in PAF1C specifically recognizes the histone H3 tail of nucleosomal substrates, stimulating H2Bub. Collectively, our biochemical results indicate that intact PAF1C is required to efficiently stimulate Rad6/Bre1-mediated H2Bub.

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The establishment of epithelial cell polarity requires the assembly of multiprotein complexes and is crucial during epithelial morphogenesis. Three scaffolding proteins, Dlgl, MPP7, and Mals3, can be assembled to form a complex that functions in the establishment and maintenance of apicobasal polarity in epithelial tissues through their L27 domains. Here we report the crystal structure of a 4‐L27‐domain complex derived from the human tripartite complex Dlg1‐MPP7‐Mals3 in combination with paramagnetic relaxation enhancement measurements. The heterotri‐mer consists of 2 pairs of heterodimeric L27 domains. These 2 dimers are asymmetric due to the large difference between the N‐ and C‐terminal tandem L27 domain of MPP7. Structural analysis combined with biochemical experiments further reveals that the loop αA‐αB and helix αB of the C‐terminal L27 domain of MPP7 play a critical role in assembling the entire tripartite complex, suggesting a synergistic tandem L27‐mediated assembling event.—Yang, X., Xie, X., Chen, L., Zhou, H., Wang, Z., Zhao, W., Tian, R., Zhang, R., Tian, C., Long, J., Shen, Y. Structural basis for tandem L27 domain‐mediated polymerization. FASEB J. 24, 4806–4815 (2010). http://www.fasebj.org