The Human Tap Nuclear RNA Factor Contains a Novel Transportin-dependent Nuclear Localization Signal That Lacks Nuclear Ex

port Signal FunctionRay Truant, Yibin Kang, and Bryan R. Cullen

871602

Shishi’s seminar

2000,11,16

Background:

‧ At least two possible protein mediators of mRNA export exist: heterogeneous nuclear ribonucleoproteins (hnRNPs) and Tap protein.

‧ Definitions of importinαβ , importin β binding domain (IBB) , nuclear localization signal (NLS) , nuclear export signal (NES) , transportin (Trn) , hnRNP A1 and M9 .

‧ M9: NLS/NES Tap-N: NLS

Background:

‧ At least two possible protein mediators of mRNA export exist: heterogeneous nuclear ribonucleoproteins (hnRNPs) and Tap protein.

‧ Definitions of importinαβ , importin β binding domain (IBB) , nuclear localization signal (NLS) , nuclear export signal (NES) , transportin (Trn) , hnRNP A1 and M9 .

‧ M9: NLS/NES Tap-N: NLS

‧ Functional domain organization of

human Tap

Transportin is necessary and sufficient for Tap-N-NLS-dependent nuclear import.

‧Competitive nuclear import assays.

‧ Transportin is sufficient for Tap-N-NLS-dependent nuclear import.

Direct binding of the Tap-N-NLS by Trn

‧In vitro binding assays.

‧Nuclear uptake of the Tap-N-NLS is sequence-specific.

Functional comparison of M9 and Tap-N-NLS

‧ Heterokaryon fusion nuclear shuttling assay.

‧HeLa cell microinjection assays.

Three types of Trn-dependent protein NLS sequences.

‧M9: specific for Trn , NLS/NES .

‧Ribosomal proteins: can also be mediate by Impβ ,NLS , no homology to M9.

‧Tap: no homology to M9 , but specific for Trn , compete with M9 , but lacks NES .Trn nuclear import factor may have evolved a specialized role to the return of mRNA export factors to the nucleus.