Nuclear import by karyopherin-βs: Recognition and inhibition

Abstract

Proteins in the Karyopherinβ family mediate the majority of macromolecular transport between the nucleus and the cytoplasm. Eleven of the 19 known human Karyopherinβs and 10 of the 14 S. cerevisiae Karyopherinβs mediate nuclear import through recognition of nuclear localization signals or NLSs in their cargos. This receptor-mediated process is essential to cellular viability as proteins are translated in the cytoplasm but many have functional roles in the nucleus. Many known Karyopherinβ-cargo interactions were discovered through studies of the individual cargos rather than the karyopherins, and this information is thus widely scattered in the literature. We consolidate information about cargos that are directly recognized by import-Karyopherinβs and review common characteristics or lack thereof among cargos of different import pathways. Knowledge of Karyopherinβ-cargo interactions is also critical for the development of nuclear import inhibitors and the understanding of their mechanisms of inhibition.

Landscape of nuclear transport receptor cargo specificity

“…However, several technical obstacles have rendered the systematic mapping of cargo–NTR relationships challenging: (i) The transient nature of cargo–NTR interactions makes them largely inaccessible to biochemical identification by NTR‐centric affinity purification; (ii) each NTR does recognize a multitude of cargos and their cargo spectrum might overlap; and (iii) cargo abundances (copy numbers per cell) span several orders of magnitude in situ . The vast majority of previous studies therefore were designed in a cargo‐centric manner and have reported transport pathways for individual cargos (for review, see, e.g., Chook & Süel, 2011; Kimura & Imamoto, 2014; Twyffels et al , 2014). Although powerful for studying the contribution of the nuclear transport system to individual cellular mechanisms, these approaches are not suitable to comprehensively chart its cargo spectrum.…”

The 21.5-kDa isoform of myelin basic protein has a non-traditional PY-nuclear-localization signal

“…Most nucleocytoplasmic transport of proteins in the cell is carried out by the Karyopherin-β (Kapβ) family of nuclear transport proteins, which recognize nuclear-localization signals (NLS) and nuclear-export signals (NES) on other proteins to carry out such transport [37][38][39]. Pedraza et al (1997) showed that not only is 21.5-kDa MBP located in the nucleus of OLGs, it is actively transported there, in an energy-as well as temperature-dependent process [31,32].…”

“…The first classical NLS is composed of 1 or 2 stretches of positively-charged amino acids, often lysine-rich [39,42]. Another type is the bipartite NLS, which consists of two basic residues, a spacer consisting of 10 residues, and another basic region consisting of at least three basic residues out of five [43].…”

“…Recently, a new diverse class of NLS motifs has been elucidated, termed the PY-NLS. This motif is defined by an N-terminal hydrophobic or basic motif, and a C-terminal RX 2-5 PY motif, where Y can also be other hydrophobic amino acids or glycine [39]. Sequence alignments from the exon-II-encoded region of 21.5-kDa MBP are shown in Fig.…”

“Programmed packaging” for gene delivery

“…Further biological analysis clarified that cytoplasmic receptors (importin families [48,49] or others [50]) recognized the NLS or the 3-demensional structure of the protein, and then form a complex that can pass through the nuclear pore complex (NPC) [51]. One of the typical NLS is derived from the SV40 T-antigen (NLS SV40 ): PKKKRKV [52].…”