Results AtPIN labelled PIN proteins in root cellsWestern blot analysis The purified AtPIN antiserum labelled one major band of approximately 68-70 kDa (Fig. 1), on western blots of wholeThe PIN-FORMED (PIN) proteins are plasma-membraneassociated facilitators of auxin transport. They are often targeted to one side of the cell only through subcellular mechanisms that remain largely unknown. Here, we have studied the potential roles of the cytoskeleton and endomembrane system in the localisation of PIN proteins. Immunocytochemistry and image analysis on root cells from Arabidopsis thaliana and maize showed that 10-30% of the intracellular PIN proteins mapped to the Golgi network, but never to prevacuolar compartments. The remaining 70-90% were associated with yet to be identified structures. The maintenance of PIN proteins at the plasma membrane depends on a BFA-sensitive machinery, but not on microtubules and actin filaments.The polar localisation of PIN proteins at the plasmamembrane was not reflected by any asymmetric distribution of cytoplasmic organelles. In addition, PIN proteins were inserted in a symmetrical manner at both sides of the cell plate during cytokinesis. Together, the data indicate that the localisation of PIN proteins is a postmitotic event, which depends on local characteristics of the plasma membrane and its direct environment. In this context, we present evidence that microtubule arrays might define essential positional information for PIN localisation. This information seems to require the presence of an intact cell wall.

This paper examines the molecular machinery involved in membrane exchange within the plant endomembrane system. A study has been undertaken on beta-COP-like proteins in plant cells using M3A5, an antibody raised against the conserved sequence of mammalian beta-COP proteins. In mammalian cells, beta-COP proteins are part of a complex named the coatomer, which probably recruits some specific areas of the endomembrane system. Immunofluorescence analyses by confocal laser scanning microscopy showed that beta-COP-like proteins marked predominantly the plant Golgi apparatus. Other proteins known to be part of a potential machinery for COPI vesicle formation (gamma-COP, beta'-COP and Arf1 proteins) were immunolocalized on the same membraneous structures as beta-COP. Moreover, beta-COP and other COPI antibodies stained the cell plate in dividing cells. It is further shown that, in maize root cells, and in contrast to observations upon mammalian cells, the drug Brefeldin A (BFA) does not induce the release of beta-COP and Arf1 proteins from the Golgi membrane into the cytosol. These data clearly demonstrate that the antibody M3A5 is a valuable marker for studies on trafficking events in plant cells. They also report for the first time the location of COP components in plant tissue at the light level, especially on a model well known for secretion, i.e. the maize root cells. They also suggest that the membrane recruitment machinery may function in a plant-specific way.

The organization and dynamics of the plant endomembrane system require both universal and plant-specific molecules and compartments. The latter, despite the growing wealth of information, remains poorly understood. From the study of an Arabidopsis thaliana male gametophytic mutant, it was possible to isolate a gene named POKY POLLEN TUBE (POK) essential for pollen tube tip growth. The similarity between the predicted POK protein sequence and yeast Vps52p, a subunit from the GARP/VFT complex which is involved in the docking of vesicles from the prevacuolar compartment to the Golgi apparatus, suggested that the POK protein plays a role in plant membrane trafficking. Genetic analysis of Arabidopsis mutants affecting AtVPS53 or AtVPS54 genes which encode putative POK partners shows a transmission defect through the male gametophyte for all lines, which is similar to the pok mutant. Using a combination of biochemical approaches and specific antiserum it has been demonstrated that the POK protein is present in phylogenetically divergent plant species, associated with membranes and belongs to a high molecular weight complex. Combination of immunolocalization studies and pharmacological approaches in different plant cells revealed that the POK protein associates with Golgi and post-Golgi compartments. The role of POK in post-Golgi endomembrane trafficking and as a member of a putative plant GARP/VFT complex is discussed.

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