Nuclear import of hnRNP A1 is mediated by a novel cellular cofactor related to karyopherin-β

Abstract

Heterogeneous nuclear ribonucleoprotein A1 contains a sequence, termed M9, that functions as a potent nuclear localization signal (NLS) yet bears no similarity to the well-defined basic class of NLSs. Here, we report the identification of a novel human protein, termed MIP, that binds M9 specifically both in vivo and in vitro yet fails to interact with non-functional M9 point mutants. Of note, the 101 kDa MIP protein bears significant homology to human karyopherin/importin-beta, a protein known to mediate the function of basic NLSs. The in vitro nuclear import of a protein substrate containing the M9 NLS was found to be dependent on provision of the MIP protein in trans. Cytoplasmic microinjection of a truncated form of MIP that retains the M9 binding site blocked the in vivo nuclear import of a substrate containing the M9 NLS yet failed to affect the import of a similar substrate bearing a basic NLS. These data indicate that nuclear import of hnRNP A1 is mediated by a novel cellular import pathway that is distinct from, yet evolutionarily related to, the pathway utilized by basic NLS sequences.

Uncoupling Kapβ2 Substrate Dissociation and Ran Binding

“…RanGppNHp interacts with the concave surface of the N-terminal arch of Kapβ2 through two contact areas: the first involves the first four HEAT repeats, which interact with the switch I and switch II regions of Ran; the second involves the central HEAT repeats 7, 8 and the acidic loop, which interact with the 'basic patch' R4-R5 region of Ran. The structure of the Kapβ2-Ran complex, in combination with previous biochemical studies using Kapβ2 deletion mutants, also allowed assignment of the substrate binding site of Kapβ2 to the concave surface of its C-terminal arch (2,10,11). The Kapβ2-Ran structure strongly suggests an allosteric mechanism for GTPase-mediated substrate dissociation.…”

Inhibition of the acetyltransferase NAT10 normalizes progeric and aging cells by rebalancing the Transportin-1 nuclear import pathway

“…(C) Volcano plots from the microarray analysis showing the number of genes for which expression was decreased (blue) or increased (red) by 1.5-fold or greater under the indicated conditions. siNAT10 and siCT treatment were transient ( 5 that defective nuclear import of TNPO1 cargo proteins including NUP153 (24) and hnRNPA1 (27) appeared to contribute to downstream phenotypic defects in HGPS cells, including abnormal nuclear pore assembly, chromatin disorganization, gene expression changes, and premature entry into senescence. Although previous studies had reported RanGTP gradient defects in cells from HGPS patients (9,10), the mechanism behind it was still unclear.…”

“…In the nucleus, RanGTP binds to TNPO1 to stimulate release of the cargo (26). Among TNPO1 cargos, the most extensively characterized is the RNA binding protein heterogeneous nuclear ribonucleoprotein 1 (hnRNPA1) (27), which functions in several processes including mRNA biogenesis and promotion of transcription factor activity (28)(29)(30). NPC protein NUP153 is also a target for TNPO1-mediated nuclear import (24).…”

HIV-1 nuclear import: in search of a leader; update 1999

“…Recent studies have shown that many members of karyopherin β family can function in nuclear import without adaptor proteins (i.e., karyopherin α) and can bind directly to their substrates (24,25). One example of such karyopherin is transportin, which mediates nuclear import of hnRNP through recognition of the M9 domain (26)(27)(28). However, in most cases the specific NLSs directly interacting with the members of karyopherin β family have yet to be elucidated.…”