Structural Basis for Recognition of Ubiquitylated Nucleosome by Dot1L Methyltransferase.

Histone H3 lysine 79 (H3K79) methylation is enriched on actively transcribed genes, and its misregulation is a hallmark of leukemia. Methylation of H3K79, which resides on the structured disk face of the nucleosome, is mediated by the Dot1L methyltransferase. Dot1L activity is part of a trans-histone crosstalk pathway, requiring prior histone H2B ubiquitylation of lysine 120 (H2BK120ub) for optimal activity. However, the molecular details describing both how Dot1L binds to the nucleosome and why Dot1L is activated by H2BK120 ubiquitylation are unknown. Here, we present the cryoelectron microscopy (cryo-EM) structure of Dot1L bound to a nucleosome reconstituted with site-specifically ubiquitylated H2BK120. The structure reveals that Dot1L engages the nucleosome acidic patch using a variant arginine anchor and occupies a conformation poised for methylation. In this conformation, Dot1L and ubiquitin interact directly through complementary hydrophobic surfaces. This study establishes a path to better understand Dot1L function in normal and leukemia cells.

Harnessing Nature's Diversity: Discovering organophosphate bioscavenger characteristics among low molecular weight proteins.

Organophosphate poisoning can occur from exposure to agricultural pesticides or chemical weapons. This exposure inhibits acetylcholinesterase resulting in increased acetylcholine levels within the synaptic cleft causing loss of muscle control, seizures, and death. Mitigating the effects of organophosphates in our bodies is critical and yet an unsolved challenge. Here, we present a computational strategy that integrates structure mining and modeling approaches, using which we identify novel candidates capable of interacting with a serine hydrolase probe (with equilibrium binding constants ranging from 4 to 120 µM). One candidate Smu. 1393c catalyzes the hydrolysis of the organophosphate omethoate (kcat/Km of (2.0 ± 1.3) × 10-1 M-1s-1) and paraoxon (kcat/Km of (4.6 ± 0.8) × 103 M-1s-1), V- and G-agent analogs respectively. In addition, Smu. 1393c protects acetylcholinesterase activity from being inhibited by two organophosphate simulants. We demonstrate that the utilized approach is an efficient and highly-extendable framework for the development of prophylactic therapeutics against organophosphate poisoning and other important targets. Our findings further suggest currently unknown molecular evolutionary rules governing natural diversity of the protein universe, which make it capable of recognizing previously unseen ligands.