The architecture of Tetrahymena telomerase holoenzyme

Telomerase adds telomeric repeats to chromosome ends using an internal RNA template and specialized telomerase reverse transcriptase (TERT), thereby maintaining genome integrity. Little is known about the physical relationships among protein and RNA subunits within a biologically functional holoenzyme. Here we describe the architecture of Tetrahymena thermophila telomerase holoenzyme determined by electron microscopy. Six of the 7 proteins and the TERT-binding regions of telomerase RNA (TER) have been localized by affinity labeling. Fitting with high-resolution structures reveals the organization of TERT, TER, and p65 in the RNP catalytic core. p50 has an unanticipated role as a hub between the RNP catalytic core, p75-p19-p45 subcomplex, and the DNA-binding Teb1. A complete in vitro holoenzyme reconstitution assigns function to these interactions in processive telomeric repeat synthesis. These studies provide the first view of the extensive network of subunit associations necessary for telomerase holoenzyme assembly and physiological function

Collection, pre-processing and on-the-fly analysis of data for high-resolution, single-particle cryo-electron microscopy

The dramatic growth in the use of cryo-electron microscopy (cryo-EM) to generate high-resolution structures of macromolecular complexes has changed the landscape of structural biology. The majority of structures deposited in the Electron Microscopy Data Bank (EMDB) at higher than 4-Å resolution were collected on Titan Krios microscopes. Although the pipeline for single-particle data collection is becoming routine, there is much variation in how sessions are set up. Furthermore, when collection is under way, there are a range of approaches for efficiently moving and pre-processing these data. Here, we present a standard operating procedure for single-particle data collection with Thermo Fisher Scientific EPU software, using the two most common direct electron detectors (the Thermo Fisher Scientific Falcon 3 (F3EC) and the Gatan K2), as well as a strategy for structuring these data to enable efficient pre-processing and on-the-fly monitoring of data collection. This protocol takes 3–6 h to set up a typical automated data collection session

Structure of a pathogenic type 3 secretion system in action

Type 3 secretion systems use 3.5-megadalton syringe-like, membrane-embedded 'injectisomes', each containing an ~800-Å-long needle complex to connect intracellular compartments of infectious bacteria and hosts. Here we identify requirements for substrate association with, transport through and exit from the injectisome of Salmonella enterica serovar Typhimurium. This guided the design of substrates that become trapped within the secretion path and enabled visualization of injectisomes in action in situ. We used cryo-EM to define the secretion path, providing a structural explanation as to why effector proteins must be unfolded during transport. Furthermore, trapping of a heterologous substrate in the needle prevents secretion of natural bacterial effectors. Together, the data reveal the path of protein secretion across multiple membranes and show that mechanisms rejecting unacceptable substrates can be undermined, and transport of bacterial effectors across an already assembled type 3 secretion system can be inhibited