Transportins 1 and 2 are redundant nuclear import factors for hnRNP A1 and HuR.
Several mRNA-binding proteins, including hnRNP A1 and HuR, contain bidirectional transport signals that mediate both their nuclear import and export. Previously, Transportin 1 (Trn1) was identified as a mediator of hnRNP A1 import, whereas the closely related protein Transportin 2 (Trn2) was shown to interact with HuR. Here we have investigated the subfamily of transportins that consists of Trn1 (or Kap beta2A) and two alternatively spliced Trn2 isoforms (Trn2a and Trn2b), also called Trn2 and Kap beta2B. The sequence differences among these proteins could alter either their cargo specificity or their response to RanGTP and thus their function as import or export receptors. Using in vitro binding assays, we show that hnRNP A1 preferentially binds Trn1 and Trn2b versus Trn2a. HuR interacts with all three transportins, as well as weakly with Imp beta. The hnRNP A1 and HuR shuttling domains, called M9 and HNS, respectively, are sufficient for these interactions. Despite small differences in the binding of HuR and hnRNP A1 to the three transportins, in vitro interaction studies performed in the presence and absence of RanQ69LGTP indicate that all three transportins most likely act as import factors for HuR and hnRNP A1. In digitonin-permeabilized HeLa cells, both M9 and HNS peptides compete for the import of recombinant hnRNP A1 and HuR, indicating that HuR and hnRNP A1 import pathways are at least partially overlapping. Possible nucleocytoplasmic shuttling mechanisms for hnRNP A1 and HuR are discussed.
Rebane
Ana
A
Department of Molecular Biophysics and Biochemistry, Howard Hughes Medical Institute, Yale University School of Medicine, New Haven, Connecticut 06536, USA.
Aab
Alar
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Steitz
Joan A
JA
D021581:Active Transport, Cell Nucleus / Q000502:physiology; D000595:Amino Acid Sequence; D000954:Antigens, Surface / Q000378:metabolism*; D002467:Cell Nucleus / Q000378:metabolism; D051959:ELAV Proteins; D000067780:ELAV-Like Protein 1; D006367:HeLa Cells; D000076245:Heterogeneous Nuclear Ribonucleoprotein A1; D034481:Heterogeneous-Nuclear Ribonucleoprotein Group A-B / Q000378:metabolism*; D006801:Humans; D008969:Molecular Sequence Data; D009154:Mutation; D017434:Protein Structure, Tertiary; D016601:RNA-Binding Proteins / Q000378:metabolism*; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism*; D020539:Sequence Analysis, Protein; D028961:beta Karyopherins / Q000378:metabolism*; D020931:ran GTP-Binding Protein / Q000235:genetics / Q000378:metabolism
D000954:Antigens, Surface; D051959:ELAV Proteins; D000067780:ELAV-Like Protein 1; C098770:ELAVL1 protein, human; D000076245:Heterogeneous Nuclear Ribonucleoprotein A1; D034481:Heterogeneous-Nuclear Ribonucleoprotein Group A-B; D016601:RNA-Binding Proteins; D018160:Receptors, Cytoplasmic and Nuclear; C108121:TNPO1 protein, human; C503621:TNPO2 protein, human; D028961:beta Karyopherins; D020931:ran GTP-Binding Protein