Figure 9.

Model of MPB2C Regulating KN1 Transport.

A hypothetical model depicting MPB2C as a KN1 transport-controlling factor. KN1 is produced in the cytosol; there, a decision is made between nuclear import (right panel), cell-to-cell transport via PD (left panel), and MPB2C-mediated association to microtubules (bottom panel). MPB2C binding to KN1 depends on a functional HD motif, which also constitutes the KN1 cell-to-cell transport signal. Once a KN1-mRNA complex enters the PD translocation pathway, the KN1 HD transport motif is modified or occupied. By this means, a KN1-mRNA complex escapes nuclear import and gains access to adjacent cells. However, aberrant or unbound KN1 molecules, not immediately entering the PD pathway, could be recognized by MPB2C. In such a scenario, KN1 could associate with MPB2C at the cytoskeleton. Here, KN1, as suggested for TMV MP (Kragler et al., 2003, Curin et al., 2007), might rapidly enter the proteasome-dependent degradation pathway. By this means, MPB2C could regulate two aspects: (1) the quality and amount of KN1 present in the shoot apical meristem, and (2) the pace of KN1-mRNA cell-to-cell transport to neighboring tissues.