|
Figure 1
Targeting of the PIN auxin efflux carrier in Arabidopsis resembles the human IRAP/GLUT4 mechanism. In human insulin responsive tissues, a vesicular cycling mechanism mediates the asymmetric localization of the GLUT4 glucose transporter on the plasma membrane. Recent studies of the localization of PIN auxin efflux carriers and protein interactions with auxin transport inhibitors suggest that a similar mechanism may regulate auxin transport in Arabidopsis. Sequence homologies and analogous functions of many of the protein components of the two systems further suggest parallel mechanisms. An external signal (hormone binding) triggers a phosphatidylinositol/Protein kinase C (Pinoid) phosphorylation cascade that activates asymmetric vesicular trafficking by: 1) causing dissociation of either an FKB506 binding protein (Twisted Dwarf) or Cyclophilin 5; 2) relocation of an inhibitory Grp ARF-GEF protein (GNOM) from an endomembrane compartment to PIP3–enriched plasma membranes; 3) recruitment of a dynamin GTPase (ADL1a) and a secretory adaptor (adaptin) complex; 4) phosphorylation of both IRAP (AtAPM1) and the Vamp2 v-snare adaptor protein. Vesicles then traffic on cytoskeletal fibers to a Munc18c (Keule) plasma membrane docking site where Vamp2 interacts with the t-snare Syntaxin 4 (Knolle) to initiate vesicle fusion. Membrane transport proteins are then recycled by endocytosis into early endosomes for redistribution to plasma membranes or, if processed sequentially by a KEXP-APP complex and PM-localized IRAP (AtAPM1), are routed to late endosomes for subsequent proteolytic degradation. For comparison, Arabidopsis proteins implicated in auxin transport are listed adjacent to their mammalian orthologs in the columns at the right of the figure. PKC, protein kinase C; AKT, protein kinase B/AKT; RCN1, root curling in NPA-1 PP2a; Vamp2, vesicle associated membrane protein 2 (v-SNARE2); IRAP, insulin responsive aminopeptidase; AtAPM1, Arabidopsis thaliana microsomal aminopeptidase; GRP1 ARF-GEF, general receptor for phosphoinositides ADP ribosylation factor-guanine nucleotide exchange factor; Munc18c, mammalian homolog of C. elegans unc18c; FKB506BP, FKB506-binding immunophilin; TWD, Twisted Dwarf; APP1, mammalian aminopeptidase P 1; AtAPP1, Arabidopsis APP1; AtKEXP, Arabidopsis KEXP neutral metallopeptidase.
|
