LAUSR

The Ser/Thr kinase BUB1 (budding uninhibited by benzimidazoles-1) is required for efficient signaling by the TGF-β receptor. Recruitment of BUB1 to the activated heteromeric TGF-β-receptor complex including SMAD2 and SMAD3, is required for ligand mediated canonical and non-canonical downstream cascades (Nyati et. al. 2015). In an effort to delineate the structure-function basis for this, we utilized in vitro kinase reactions followed by mass spectrometry to demonstrate that TGFBR2 phosphorylates BUB1 at Serine 318. To elucidate the functional significance of TGFBR2 mediated phosphorylation of BUB1, S318 was substituted with alanine (S318A) or aspartic acid (S318D). Co-immunoprecipitation studies of these phospho-deficient or phospho-mimicking BUB1 mutants with TGFBR1, and SMAD2 revealed that the S318A mutant interacted more efficiently to TGFBR1 and SMAD2 compared to the S318D BUB1. To investigate if the Ser318 residue was involved in mediating BUB1 interaction with TGF-β signaling components, we utilized deletion mutants of BUB1. Our findings reveal that the N-terminal (1-241) and C-terminal (482-723) domains of BUB1 independently interact with SMAD2, SMAD3, TGFBR1, or TGFBR2. The middle domain (241-482) having a S318A mutation interacted efficiently with TGFBR2 while the S318D mutant did not. Additionally, the 241-482 S318D mutant exhibited an increased interaction with SMAD2. Over-expression of 241-482 S318D resulted in a decrease in TGFBR1-TGFBR2 interaction as well as TGFBR1-SMAD2 interaction, suggesting that TGFBR2 mediated BUB1 phosphorylation at S318 maybe the trigger for the dissociation of the activated receptor complex, and therefore a key regulator of TGF-β signaling. Taken together, we show that TGFBR2 phosphorylates BUB1 at Ser318 and propose a model with the following steps: (i) BUB1 is recruited to TGFBR1-TGFBR2 complex in response to ligand, (ii) BUB1 participates in the recruitment of SMAD2/3 to the receptor, (iii) TGFBR2 phosphorylates BUB1 at S318, which triggers the disassembly of the activated complex. Citation Format: Shyam Nyati, Brandon Gregg, Jiaqi Xu, Grant Young, Lauren Kimmel, Nyati Mukesh, Dipankar Ray, Hongtao Yu, Alnawaz Rehemtulla. TGFBR2 mediated phosphorylation of BUB1 at Ser-318 is required for transforming growth factor-β signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3430.