Reviews & Analysis

MFSD2A mediates uptake of the essential fatty acid DHA across the blood–brain barrier. Separately, via interactions with syncytin-2, MFSD2A contributes to the formation of the mother–fetus placental boundary. Cryo-EM analysis of a human MFSD2A–syncytin-2 complex provides new insights into how MFSD2A performs these dual roles.

The central apparatus regulates the beating of motile cilia. High-resolution structures of the almost complete central apparatus are now reported in two separate studies, shedding light on the mechanism of ciliary beating and marking a new era in our molecular understanding of cilia architecture and function.

The CRISPR–Cas enzyme Cas9 faces the challenge of identifying a specific nucleotide sequence within double-stranded DNA. New cryo-EM and biochemical studies show that in the earliest steps of binding, Cas9 bends the DNA and promotes unwinding of two base pairs, enabling it to efficiently scan the sequence of this critical region.

Cryo-EM analysis reveals the mechanism by which chromatin is compacted at the centromere by the H3 histone variant CENP-N. Intriguingly, despite the structural differences between CENP-N and linker H1 histones, both appear to similarly compact higher-order nucleosome structures.

Eukaryotes possess several clamp loaders comprising four common subunits and a fifth subunit unique to each complex. The RFC-A–E clamp loader loads the PCNA clamp at 3′-recessed structures for DNA replication. However, swapping a single subunit, Rad24, for RFC-A yields a clamp loader that prefers the 911 clamp and 5′-recessed DNA. Three new studies reveal detailed views of the clamp loading reaction and provide insights into substrate preferences of each loader.

The molecular mechanisms by which a few molecules of the long non-coding RNA Xist silence genes on the entire X chromosome are poorly understood. New evidence suggests that dimeric foci of Xist seed the formation of large protein assemblies that contain a wide spectrum of proteins, such as SPEN (SHARP), CIZ1, CELF, PTBP1 and components of Polycomb repressive complexes 1 and 2. These assemblies, each of which may contain hundreds to thousands of molecules of proteins, extend spatially beyond each focus of Xist, which explains how this long non-coding RNA triggers silencing across an entire chromosome.

The class II phosphatidylinositol 3-kinases (PI3Ks) serve important roles in diverse cellular processes. The lab of Volker Haucke has now determined high-resolution structures of mouse PI3KC2α in both active and inactive conformations, elucidating the autoregulatory mechanism of class II PI3K activation.

Growing evidence suggests that many ribosome-targeting antibiotics inhibit protein synthesis context specifically, which has important implications for drug development. New work reveals the structural basis of context-specific action of the classic translation inhibitor chloramphenicol and the oxazolidinones linezolid and radezolid.

An elegant cryo-EM and biophysical study unveils the conformational changes of the E. coli MutS mismatch repair factor. This provides a nice follow-up to the recent report from the same group characterizing how MutS scans DNA without initiating repair on correctly base-paired DNA and recruits MutL upon encountering a mismatch.

Retinal photoreceptor cyclic nucleotide-gated (CNG) ion channels convert light signals to electrical signals in the eye. Their structures have been solved at ~3 Å resolution by cryo-EM; the asymmetric subunit assembly of heteromeric CNG channels produces a unique ion-permeation pathway with an unusual gating apparatus that indicates distinct subunit contributions for ligand-dependent channel activation.

Iboxamycin (IBX) is a new oxepanoprolinamide antibiotic based on clindamycin. Crystal structures of IBX in complex with bacterial ribosomes uncover the structural mechanism of its activity against multidrug-resistant pathogens and reveal key interactions with tRNAs and 23S rRNA, including resistance-conferring rRNA methylations.

New work from Guberovic et al. sheds light on the evolution of the histone variant macroH2A1.1 and its relationship with the NAD⁺-using poly(ADP-ribose) polymerase PARP1. Their study shows that macroH2A1.1 has been a nuclear regulator of NAD⁺ flux as far back evolutionarily as pre-metazoan protists, but has been loosening the reins on PARP1, thus expanding PARP1’s cellular roles.

Transcription of eukaryotic protein-coding genes involves co-activator complexes, including TFIID and SAGA. A new study has determined the first high-resolution cryo-EM structure of the human SAGA complex, and has implications for defining SAGA function during multiple stages of eukaryotic transcription.