Auxins induce clustering of the auxin-binding protein at the surface of maize coleoptile protoplasts.

Abstract

The predominant localization of the major auxin-binding protein (ABP1) of maize is within the lumen of the endoplasmic reticulum. Nevertheless, all the electrophysiological evidence supporting a receptor role for ABP1 implies that a functionally important fraction of the protein must reside at the outer face of the plasma membrane. Using methods of protoplast preparation designed to minimize proteolysis, we report the detection of ABP at the surface of maize coleoptile protoplasts by the technique of silverenhanced immunogold viewed by epipolarization microscopy. We also show that ABP clusters following auxin treatment and that this response is temperature-dependent and auxinspecific.The hormone auxin plays a pivotal role in regulating plant growth and development (1). Auxin stimulation implies that the hormone must be recognized (hormone binding) and that its perception must be converted into a physiological response (signal transduction). Many early reports have provided evidence for the binding of auxin to plant membranes, especially the endoplasmic reticulum (ER) (for review, see ref.2). Subsequent work (reviewed in refs. 1 and 3) has resulted in the isolation and characterization of the major protein (ABP1) responsible for auxin binding in maize (Zea mays) coleoptiles. ABP (auxin-binding protein) is a dimeric protein of Mr 44,000 (4-6) which binds either one (4) or two (7) moles of auxin per dimer. Sequencing of cDNA clones for maize ABP (7-10) has indicated a protein of 163 amino acids, 38 of which represent a typical hydrophobic signal peptide at the amino terminus. In addition, ABP has a Lys-Asp-Glu-Leu (KDEL) sequence at its carboxyl terminus and has a single, high-mannose glycan, which is sensitive to endoglycosidase H digestion (5, 11). These are features of proteins that are retained within the lumen of the ER (12) and thus conform with the earlier binding studies on microsomal membranes.Although the biochemical characteristics of maize ABP are indicative of an ER-resident protein, a number of observations strongly suggest that some of the total cellular ABP is also localized at the cell surface. It has been established both by classical (microelectrode impalement; refs. 13 and 14) and whole-cell patch-clamp (15) electrophysiological methods that auxin causes an increase in HI current at the plasma membrane (PM). Since this effect is blocked by antibodies against H+-ATPase (13) and is further enhanced by the fungal toxin fusicoccin (15), it has been considered that it reflects an activation of the PM-localized H+-ATPase. Whereas polyclonal antibodies raised against maize ABP (5, 16) prevent these auxin effects (reviewed in ref. 17), antibodies raised against a synthetic peptide corresponding to the putative auxin binding site of ABP induce auxin-like electrophysiological changes at the plasma membrane (15,18). Two further obser-The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 ...

AUXIN BINDING PROTEIN1: The Outsider

“…The only effect of auxin on ABP1 localization reported so far is a clustering of ABP1 at the outer membrane of maize coleoptile protoplasts after auxin treatment (Diekmann et al, 1995). Interestingly, Jones and Herman (1993) reported that auxin starvation of cell cultures seemed to increase ABP1 secretion into the medium, but no other study followed up on this observation.…”

“…Small amounts of ABP1 have been visualized by immunocytochemistry close to the plasma membrane on the inner (cytoplasmic) side and on the outer side, extending into the cell wall (Jones and Herman, 1993;Diekmann et al, 1995). Henderson et al (1997) failed to detect ABP1 in the cell wall, but they cautioned that ABP1 amounts might be at the detection limit of direct visualization.…”

RAC/ROP GTPases and Auxin Signaling

“…These include the observations that overexpression of ABP1 mediates auxin-dependent cell expansion (Jones et al, 1998), embryo lethality in Arabidopsis abp1 knockout mutants (Chen et al, 2001), compromised auxin transport, and auxinregulated gene expression and other cellular and developmental defects in abp1/ABP1 heterozygotes (Effendi et al, 2011) and conditionally ABP1-suppressed plants (Braun et al, 2008;Tromas et al, 2009). Immunodetection and quantitative glycan analysis of ABP1 showed that a small portion of this predominantly ER-located protein could be detected on the cell surface (Jones and Herman, 1993;Henderson et al, 1997) and that auxin can induce its clustering on the protoplast surface (Diekmann et al, 1995). In showing that ABP1 acts as a signal mediator for auxin to activate RAC/ROP-regulated epidermal cell patterning, Xu et al (2010) proposed that it may act as a cell surface regulator for auxin-mediated pavement cell interdigitation.…”

ABP1 is required for organized cell elongation and division in Arabidopsis embryogenesis

“…For example, it is possible that ABP1 has some ligand-regulated chaperone role. ABP1 is found predominantly in the lumen of the endoplasmic reticulum, although it is certainly at detectable levels throughout the endomembrane system (Jones and Herman 1993;Henderson et al 1997) and at the plasma membrane/cell wall interface (Deikman et al 1995). Our working hypothesis is that ABP1 regulates the direction or flux of wall materials.…”