Filamins are large proteins that cross-link actin filaments and connect to other cellular components. The C-terminal rod 2 region of FLNa (filamin A) mediates dimerization and interacts with several transmembrane receptors and intracellular signalling adaptors. SAXS (small-angle X-ray scattering) experiments were used to make a model of a six immunoglobulin-like domain fragment of the FLNa rod 2 (domains 16-21). This fragment had a surprising three-branched structural arrangement, where each branch was made of a tightly packed two-domain pair. Peptides derived from transmembrane receptors and intracellular signalling proteins induced a more open structure of the six domain fragment. Mutagenesis studies suggested that these changes are caused by peptides binding to the CD faces on domains 19 and 21 which displace the preceding domain A-strands (18 and 20 respectively), thus opening the individual domain pairs. A single particle cryo-EM map of a nine domain rod 2 fragment (domains 16-24), showed a relatively compact dimeric particle and confirmed the three-branched arrangement as well as the peptide-induced conformation changes. These findings reveal features of filamin structure that are important for its interactions and mechanical properties.

In the process of protein synthesis, the small (40S) subunit of the eukaryotic ribosome is recruited to the capped 5 end of the mRNA, from which point it scans along the 5 untranslated region in search of a start codon. However, the 40S subunit alone is not capable of functional association with cellular mRNA species; it has to be prepared for the recruitment and scanning steps by interactions with a group of eukaryotic initiation factors (eIFs). In budding yeast, an important subset of these factors (1, 2, 3, and 5) can form a multifactor complex (MFC). Here, we describe cryo-EM reconstructions of the 40S subunit, of the MFC, and of 40S complexes with MFC factors plus eIF1A. These studies reveal the positioning of the core MFC on the 40S subunit, and show how eIF-binding induces mobility in the head and platform and reconfigures the head-platform-body relationship. This is expected to increase the accessibility of the mRNA channel, thus enabling the 40S subunit to convert to a recruitment-competent state.posttranscriptional gene expression ͉ protein synthesis ͉ ribosome structure E lucidation of the mechanisms underlying ribosome function and protein synthesis remains one of the major challenges of molecular biology. Recent progress in structural analysis of bacterial ribosomes has provided insight into the likely modes of action of core functional centers, including those for decoding and peptidyl transferase, and the tRNA-binding sites (1). Analogous core structural features are clearly shared by the eukaryotic counterpart, but there is much less structural and mechanistic information available that is specific to the eukaryotic ribosome. This limits our understanding of the process of translation initiation, the step where major differences between the prokaryotic and eukaryotic systems are evident (2).The small ribosomal subunit in both prokaryotes (30S) and eukaryotes (40S) is responsible for controlling base pairing between the tRNA anticodon and each mRNA codon during protein synthesis. However, unlike its prokaryotic (30S) counterpart, the eukaryotic 40S subunit does not locate directly to the position of the mRNA AUG codon where protein synthesis begins. Instead, recruitment onto cellular mRNAs generally occurs via the capped 5Ј end. Because the AUG start codon can be located many hundreds of nucleotides downstream, the 40S subunit then has to translocate to reach the initiation site (3) [see supporting information (SI) Fig. 5]. During this processive, sequence-independent scanning phase, the 40S subunit manifests some characteristics that appear to be similar to those of a molecular motor (2, 4).The eukaryotic 40S subunit alone is incapable of stable recruitment onto the capped 5Ј ends of cellular mRNA molecules. Its role in translation initiation depends on a large number of eukaryotic initiation factors [the eIFs (5)]. According to the current classification, 11 distinct eIFs (including eIF2B, a guanine nucleotide exchange factor) are involved in (steady-state) translation initiation. There has bee...

The catalytic activity of Src-family kinases is regulated by association with its SH3 and SH2 domains. Activation requires displacement of intermolecular contacts by SH3/SH2 binding ligands resulting in dissociation of the SH3 and SH2 domains from the kinase domain. To understand the contribution of the SH3-SH2 domain pair to this regulatory process, the binding of peptides derived from physiologically relevant SH2 and SH3 interaction partners was studied for Lck and its relative Fyn by NMR spectroscopy. In contrast to Fyn, activating ligands do not induce communication between SH2 and SH3 domains in Lck. This can be attributed to the particular properties of the Lck SH3-SH2 linker which is shown to be extremely flexible thus effectively decoupling the behavior of the SH3 and SH2 domains. Measurements on the SH32 tandem from Lck further revealed a relative domain orientation that is distinctly different from that found in the Lck SH32 crystal structure and in other Src kinases. These data suggest that flexibility between SH2 and SH3 domains contributes to the adaptation of Src-family kinases to specific environments and distinct functions.