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Albany 2015:Book of Abstracts

Albany 2015
Conversation 19
June 9-13 2015
©Adenine Press (2012)

Molecular basis for nucleo-cytoplasmic transport of tRNA by Exportin-t

Export of mature, fully processed tRNA molecules from nucleus to cytoplasm is carried out by Exportin-t (Xpot) protein in mammals (Los1p homolog in S. cerevisiae). It belongs to a family of nuclear import/export proteins called Karyopherins, formed by a repeating motif of 35-40 amino acids called HEAT repeats. The export and subsequent release of tRNA cargo by Xpot involves Ran-GTP hydrolysis in the cytoplasm which is associated with Xpot. Despite the availability of crystal structures of nuclear and cytosolic forms of Xpot, the details regarding the sequential events leading to tRNA release and conformational change in Xpot remains unclear. We have studied a range of molecular complexes including free Xpot protein and intermediate state complexes bound either to Ran or tRNA, to understand the gross structural motions in Xpot after cargo release and identified various molecular determinants responsible for cargo binding. A combination of classical all atom MD and accelerated MD simulations have been performed to study the molecular complexes involving Xpot. This combinatorial approach provided a statistically reliable estimate of the conformational space explored by Xpot in cargo free forms. The overall conformational change in Xpot due to cargo release was attributed to a highly fluctuating C-terminal region. Structural hinges, which facilitated Xpot opening from nuclear to cytosolic state were established at HEAT repeats 9/10 and 12/13. In ternary Xpot-Ran-tRNA complexes (GTP/GDP), immediate effect of GTP hydrolysis was characterized by loss of contacts between the tRNA TΨC stem-loop/D-loop region and Heats 9/16-19 in Xpot. HEAT repeats 9 and 17 played opposite roles in binary Xpot-Ran and ternary Xpot-Ran-tRNA complex. Interaction between Xpot and Ran at H9 were considerably more in binary form as compared to ternary complex and vice-versa for H17, indicating that these two HEATs might play a critical role in transition from free Xpot to tRNA bound forms.

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Asmita Gupta
Manju Bansal*

Molecular Biophysics Unit
Indian Institute of Science
Bangalore 560012 India

mb@mbu.iisc.ernet.in