The best understood system for the transport of macromolecules between the cytoplasm and the nucleus is the classical nuclear import pathway. In this pathway, a protein containing a classical basic nuclear localization signal (NLS) is imported by a heterodimeric import receptor consisting of the ␤-karyopherin importin ␤, which mediates interactions with the nuclear pore complex, and the adaptor protein importin ␣, which directly binds the classical NLS. Here we review recent studies that have advanced our understanding of this pathway and also take a bioinformatics approach to analyze the likely prevalence of this system in vivo. Finally, we describe how a predicted NLS within a protein of interest can be confirmed experimentally to be functionally important.In eukaryotic cells, the genetic material and transcriptional machinery of the nucleus are separated from the translational machinery and metabolic systems of the cytoplasm by the nuclear envelope. This segregation facilitates the precise regulation of cellular processes such as gene expression (1), signal transduction (2), and cell cycle progression (3) through selective regulation of bidirectional transport between the nucleus and the cytoplasm. However, this physical separation also necessitates the existence of molecular machinery that specifically recognizes cargo in one compartment, translocates it through the nuclear pore, and releases it in the other compartment. Nuclear transport systems of this kind were first proposed when a nuclear targeting signal in the simian virus 40 (SV40) 2 large T antigen was characterized more than 20 years ago (4, 5). Since then, several pathways for nucleocytoplasmic transport have been described, of which the classical nuclear import pathway is the best characterized. The integration of detailed structural information on the components of the pathway, whole genome surveys, and extensive molecular analysis has generated powerful insight into the crucial interactions that underlie this pathway. Here we review recent studies that have defined key aspects of the cargo/import receptor interaction in the classical nuclear import cycle and present results of a bioinformatics-based assessment of the likely prevalence of this system within the model eukaryotic organism, Saccharomyces cerevisiae. Overview of Nucleocytoplasmic TransportTransport of macromolecules into and out of the nucleus occurs through large, proteinaceous structures called nuclear pore complexes (NPCs) (6 -9). Nuclear pore complexes allow passive diffusion of ions and small proteins (Ͻ40 kDa) but restrict passage of larger molecules to those containing an appropriate targeting signal (10, 11). The pores are constructed from a class of proteins called "nucleoporins," a subset of which contains a tandem series of phenylalanine-glycine (FG) repeats that line the central transport channel of the pore (8,(12)(13)(14).The active transport of macromolecular cargo between the cytoplasmic and nuclear compartments is facilitated by specific soluble carrier proteins. Th...

The yeast nucleoporin Nup2p is associated primarily with the nuclear basket of nuclear pore complexes and is required for ef®cient importin-a:b-mediated nuclear protein import as well as ef®cient nuclear export of Kap60p/importin-a. Residues 1±51 of Nup2p bind tightly to Kap60p and are required for Nup2p function in vivo. We have determined the 2.6 A Ê resolution crystal structure of a complex between this region of Nup2p and the armadillo repeat domain of Kap60p. Nup2p binds along the inner concave groove of Kap60p, but its interaction interface is different from that employed for nuclear localization signal (NLS) recognition although there is some overlap between them. Nup2p binds Kap60p more strongly than NLSs and accelerates release of NLSs from Kap60p. Nup2p itself is released from Kap60p by Cse1p:RanGTP only in the presence of the importin-b binding (IBB) domain of Kap60p. These data indicate that Nup2p increases the overall rate of nuclear traf®cking by coordinating nuclear import termination and importin recycling as a concerted process.

Nuclear localization signals (NLSs) are amino acid sequences that target cargo proteins into the nucleus. Rigorous characterization of NLS motifs is essential to understanding and predicting pathways for nuclear import. The best-characterized NLS is the classical NLS (cNLS), which is recognized by the cNLS receptor, importin-α. cNLSs are conventionally defined as having one (monopartite) or two clusters of basic amino acids separated by a 9-12 aa linker (bipartite). Motivated by the finding that Ty1 integrase, which contains an unconventional putative bipartite cNLS with a 29 aa linker, exploits the classical nuclear import machinery, we assessed the functional boundaries for linker length within a bipartite cNLS. We confirmed that the integrase cNLS is a bona fide bipartite cNLS, then carried out a systematic analysis of linker length in an obligate bipartite cNLS cargo, which revealed that some linkers longer than conventionally defined can function in nuclear import. Linker function is dependent on the sequence and likely the inherent flexibility of the linker. Subsequently, we interrogated the Saccharomyces cerevisiae proteome to identify cellular proteins containing putative long bipartite cNLSs. We experimentally confirmed that Rrp4 contains a bipartite cNLS with a 25 aa linker. Our studies show that the traditional definition of bipartite cNLSs is too restrictive and linker length can vary depending on amino acid composition.

Gene rearrangements resulting in the aberrant activity of tyrosine kinases have been identified as drivers of oncogenesis in a variety of cancers. The tropomyosin receptor kinase (TRK) family of tyrosine receptor kinases is emerging as an important target for cancer therapeutics. The TRK family contains three members, TRKA, TRKB, and TRKC, and these proteins are encoded by the genes NTRK1, NTRK2, and NTRK3, respectively. To activate TRK receptors, neurotrophins bind to the extracellular region stimulating dimerization, phosphorylation, and activation of downstream signaling pathways. Major known downstream pathways include RAS/MAPK/ERK, PLCγ, and PI3K/Akt. While being rare in most cancers, TRK fusions with other proteins have been well-established as oncogenic events in specific malignancies, including glioblastoma, papillary thyroid carcinoma, and secretory breast carcinomas. TRK protein amplification as well as alternative splicing events have also been described as contributors to cancer pathogenesis. For patients harboring alterations in TRK expression or activity, TRK inhibition emerges as an important therapeutic target. To date, multiple trials testing TRK-inhibiting compounds in various cancers are underway. In this review, we will summarize the current therapeutic trials for neoplasms involving NTKR gene alterations, as well as the promises and setbacks that are associated with targeting gene fusions.

The structure of solenoid proteins facilitates a higher degree of flexibility than most folded proteins. In importin-β, a nuclear import factor built from 19 tandem HEAT repeats, flexibility plays a crucial role in allowing interactions with a range of different partners. We present a comprehensive analysis of importin-β flexibility based on a number of different approaches. We determined the crystal structure of unliganded Saccharomyces cerevisiae importin-β (Kap95) to allow a quantitative comparison with importin-β bound to different partners. Complementary mutagenesis, small angle X-ray scattering and molecular dynamics studies suggest that the protein samples several conformations in solution. The analyses suggest the flexibility of the solenoid is generated by cumulative small movements along its length. Importin-β illustrates how solenoid proteins can orchestrate protein interactions in many cellular pathways.