No abstract

During their final maturation in the cytoplasm, pre-60S ribosomal particles are converted to translation-competent large ribosomal subunits. Here, we present the mechanism of peptidyltransferase centre (PTC) completion that explains how integration of the last ribosomal proteins is coupled to release of the nuclear export adaptor Nmd3. Single-particle cryo-EM reveals that eL40 recruitment stabilises helix 89 to form the uL16 binding site. The loading of uL16 unhooks helix 38 from Nmd3 to adopt its mature conformation. In turn, partial retraction of the L1 stalk is coupled to a conformational switch in Nmd3 that allows the uL16 P-site loop to fully accommodate into the PTC where it competes with Nmd3 for an overlapping binding site (base A2971). Our data reveal how the central functional site of the ribosome is sculpted and suggest how the formation of translation-competent 60S subunits is disrupted in leukaemia-associated ribosomopathies.

Summary The human EAG1 potassium channel belongs to the superfamily of KCNH voltage-gated potassium channels that have roles in cardiac repolarization and neuronal excitability. EAG1 is strongly inhibited by Ca2+-calmodulin (CaM) through a mechanism that is not understood. We determined the binding properties of CaM with each one of three previously identified binding sites (BDN, BDC1 and BDC2), analyzed binding to protein stretches that include more than one site, and determined the effect of neighboring globular domains on the binding properties. The determination of the crystal structure of CaM bound to BDC2 shows the channel fragment interacting with only the C-lobe of calmodulin and adopting an unusual bent conformation. Based on this structure and on a functional and biochemical analysis of mutants, we propose a model for the mechanism of inhibition where the local conformational change induced by CaM binding at BDC2 is at the basis of channel modulation.