Microtubules are filamentous polymers essential for cell viability. Microtubule plus-end tracking proteins (+TIPs) associate with growing microtubule plus ends and control microtubule dynamics and interactions with different cellular structures during cell division, migration, and morphogenesis. EB1 and its homologs are highly conserved proteins that play an important role in the targeting of +TIPs to microtubule ends, but the underlying molecular mechanism remains elusive. By using live cell experiments and in vitro reconstitution assays, we demonstrate that a short polypeptide motif, Ser-x-Ile-Pro (SxIP), is used by numerous +TIPs, including the tumor suppressor APC, the transmembrane protein STIM1, and the kinesin MCAK, for localization to microtubule tips in an EB1-dependent manner. Structural and biochemical data reveal the molecular basis of the EB1-SxIP interaction and explain its negative regulation by phosphorylation. Our findings establish a general "microtubule tip localization signal" (MtLS) and delineate a unifying mechanism for this subcellular protein targeting process.

TDP-43 encodes an alternative-splicing regulator with tandem RNA-recognition motifs (RRMs). The protein regulates cystic fibrosis transmembrane regulator (CFTR) exon 9 splicing through binding to long UG-rich RNA sequences and is found in cytoplasmic inclusions of several neurodegenerative diseases. We solved the solution structure of the TDP-43 RRMs in complex with UG-rich RNA. Ten nucleotides are bound by both RRMs, and six are recognized sequence specifically. Among these, a central G interacts with both RRMs and stabilizes a new tandem RRM arrangement. Mutations that eliminate recognition of this key nucleotide or crucial inter-RRM interactions disrupt RNA binding and TDP-43-dependent splicing regulation. In contrast, point mutations that affect base-specific recognition in either RRM have weaker effects. Our findings reveal not only how TDP-43 recognizes UG repeats but also how RNA binding-dependent inter-RRM interactions are crucial for TDP-43 function.

Odorants are transmitted by small hydrophobic molecules that cross the aqueous sensillar lymph surrounding the dendrites of the olfactory neurons to stimulate the olfactory receptors. In insects, the transport of pheromones, which are a special class of odorants, is mediated by pheromone-binding proteins (PBPs), which occur at high concentrations in the sensillar lymph. The PBP from the silk moth Bombyx mori (BmPBP) undergoes a pH-dependent conformational transition between the forms BmPBP A present at pH 4.5 and BmPBP B present at pH 6.5. Here, we describe the NMR structure of BmPBP A , which consists of a tightly packed arrangement of seven ␣-helices linked by well defined peptide segments and knitted together by three disulfide bridges. A scaffold of four ␣-helices that forms the ligand binding site in the crystal structure of a BmPBP-pheromone complex is preserved in BmPBP A . The C-terminal dodecapeptide segment, which is in an extended conformation and located on the protein surface in the pheromone complex, forms a regular helix, ␣7, which is located in the pheromone-binding site in the core of the unliganded BmPBP A . Because investigations by others indicate that the pH value near the membrane surface is reduced with respect to the bulk sensillar lymph, the pH-dependent conformational transition of BmPBP suggests a novel physiological mechanism of intramolecular regulation of protein function, with the formation of ␣7 triggering the release of the pheromone from BmPBP to the membrane-standing receptor. Male moths have an exquisitely sensitive olfactory system capable of detecting over great distances single molecules of pheromones emitted by the female, and they are capable of distinguishing highly selectively between closely similar compounds or isomers (1). These capabilities of male moths are because of olfactory sensory organs that consist of large branched antennae and can readily be isolated for biochemical characterization or electrophysiological recordings (2). The surface of the antennae is covered with hairlike protrusions composed of cuticle, which form a sheath (sensillum) surrounding one or several dendrite endings from olfactory neurons. The dendrites in each sensillum are bathed in a special fluid, the sensillum lymph. Odorant molecules gain access to the dendritic membrane through pores penetrating the sensillum wall and then crossing the sensillar lymph to interact with G proteincoupled olfactory receptors, which form a large eukaryotic protein family. Upon interaction with the olfactory receptors, a cascade of events leads to a change of the membrane potential (3), resulting in opening of ion channels and depolarization.Common odorants are hydrophobic molecules that are poorly soluble in aqueous media, such as the sensillar lymph. Odorants overcome this barrier by binding to odorant-binding proteins (OBPs). OBPs are present at high concentrations in the lymph (up to 10 mM), bind the odorant, and transport it from the sensillum pore wall to the receptor at the dendritic membrane. It is ...

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