The Minus-End-Directed Kinesin OsDLK Shuttles to the Nucleus and
Modulates the Expression of Cold-Box Factor 4
Abstract
The transition to terrestrial plants was accompanied by a progressive
loss of microtubule minus end-directed dynein motors. Instead, the
minus-end directed class-XIV kinesins expanded considerably, probably
related to novel functions. One of these motors, OsDLK (Dual
Localisation Kinesin from rice) decorates cortical microtubules but
moves into the nucleus in response to cold stress. This analysis of
loss-of-function mutants in rice indicates that OsDLK participates in
cell elongation during development. Since OsDLK harbours both, a nuclear
localisation signal, and a putative leucin zipper, we asked, whether the
cold-induced import of OsDLK into the nucleus might correlate with a
specific DNA binding. Conducting a DPI-ELISA screen with recombinant
OsDLKT (lacking the motor domain), we identified the Opaque2 motif as
most promising candidate. This motif is present in the promoter of
NtAvr9/Cf9, the tobacco homologue of Cold Box Factor 4, a transcription
factor involved in cold adaptation. A comparative study revealed that
the cold-induced accumulation of NtAvr9/Cfp9 was specifically quelled in
transgenic BY-2 cells overexpressing OsDLK-GFP. These findings are
discussed as a working model, where, in response to cold stress, OsDLK
partitions from cortical microtubules at the plasma membrane into the
nucleus and specifically modulates the expression of genes involved in
cold adaptation.