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Regulation of Cell-To-Cell Transport of RNA and Proteins via Plasmodesmata
In plants homeotic proteins involved in meristem initiation and/or maintenance such as KNOTTED1 (KN1) or which confer and maintain floral meristem identity such as LEAFY are thought to move like viral movement proteins to adjacent cells via intercellular pores formed by plasmodesmata. A number of studies suggest that specific and regulated cell-to-cell transport of proteins and RNA molecules depends on cellular plasmodesmal pathway receptors. Homeotic transcription factors function within the nucleus, thus, a decision between non-cell-autonomy versus nuclear import has to be made within the cytoplasm. Next, actively transported transcription factors have to interact with specific receptors regulating access to the intercellular transport machinery established by plasmodesmata, which in turn transfers the non-cell-autonomous proteins to neighboring cells via the plasmodesmatal pores. However, limited knowledge is available regarding interaction partners of non-cell-autonomous transcription factors such as KN1-binding proteins and their functional role(s) in cellular distribution.
Recent results indicate that entry into the plasmodesmal transport pathway can be negatively regulated by a novel microtubules-associated protein named MPB2C. Structurally and functionally distinct proteins such as the tobacco mosaic virus movement protein (TMV-MP), KNOTTED1 (KN1) as well as the A. thaliana KN1 homologue SHOOT MERISTEMLESS (STM) interact with MPB2C. Ample presence of the MPB2C prevents cell-to-cell movement of the homeobox transcription factor KN1 and TMV-MP. In addition, we isolated a novel KN1/STM binding protein, KNB36, which is also transported from cell to cell and binds to MPB2C. To exclude that KN1 cell-to-cell transport is simply impaired due to the overexpression of an interacting protein we tested the effect of KNB36 on KN1 transport. In microinjection experiments KNB36 has no effect KN1 cell-to-cell transport. These observations suggest that MPB2C binds a range of structurally distinct proteins and specifically decides/regulates their intracellular distribution and, consequently, also access to the non-cell-autonomous transport pathway established by plasmodesmata.
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