Auxin-binding Sites of Maize Coleoptiles Are Localized on Membranes of the Endoplasmic Reticulum

Abstract

Sites in maize (Zea mays L.) coleoptile homogenates that reversibly bind naphthalene-l-acetic acid with high affinity and may represent receptor sites for auxins are located primarily on ceilular membranes that show the enzymic and buoyant density characteristics of membranes of the rough endoplasmic reticulum. The sites remain attached to the endoplasmic reticulum (ER) (13) similarly reported on membrane-bound binding sites for the auxin transport inhibitors N-1-naphthylphthalamic acid and morphactins. The present paper shows the behavior, in density gradient fractionation, of the membranous particles that bear these sites. The results indicate that most of the NAA3-binding sites are located on membranes of the RER of these cells while the NPA-binding sites are located on a different membrane system, probably the plasma membrane. This same conclusion was recently published (19), based on a more limited cell fractionation procedure, in work that was initiated while the presently described study was in progress. MATERIALS AND METHODSPlant Material. Seedlings of Zea mays L., hybrid WF9 x Bear 38, were raised, harvested for coleoptiles, and homogenized, as described previously (24). The homogenization medium was as specified (24) Differential Centrifugation. The homogenate was centrifuged in an angle rotor (Sorvall SS-34 up to 41,000g, Spinco type 65 for higher forces) in the appropriate centrifuge operated at 0 C. Pellets were washed by resuspension in 0.25 M sucrose containing 10 mm tris (pH 7), 1 mm EDTA, and 0.1 mM MgCl2. An aliquot was removed for enzyme assays and the remainder of each particle fraction was recentrifuged exactly as in the centrifugation by which it had been previously pelleted. Each pellet was resuspended, using a Potter-Elvehjem type homogenizer, using 1.8 ml of standard binding assay medium (pH 5.5) (24) per g fresh weight of tissue from which the particles were derived. This suspension was used for binding assays.Rate Zonal Centrifugation. The homogenate was centrifuged 10 min at 5OOg to remove any debris, then 10 ml of it was layered over each of two linear 30-ml gradients of 10 to 25% (w/ w) sucrose made up in 10 mm tris (pH 7), containing 1 mM EDTA, 0.1 mM MgC92, and 1 mm KCI (gradient basal medium).The gradients, each contained in a cellulose nitrate tube (3.1 x 8.9 cm), were centrifuged 30 min at 9,000 rpm in the Spinco SW 25.2 rotor (10,OOOg at ray), and fractionated from the bottom into 4.2-ml fractions. Corresponding fractions from the two gradients were combined, centrifuged 20 min at 100,000g, the pellet was resuspended in 3.5 ml of 10% sucrose containing the same constituents as the gradient media just listed, and used for the assays to be described.Isopycnic Centrifugation. The majority of the mitochondria, plastids, and larger particles were removed by preliminary centrifugation of the homogenate in an angle rotor for 10 min at 6780g. Particles in the supernatant from this centrifugation were concentrated by layering 49 ml of it over layers of 6 ml 45% (w/ w) sucro...

AUXIN BINDING PROTEIN1: The Outsider

“…The biochemical properties of this binding activity were characterized Ray, 1977;Venis, 1977), but it was not until 1985 that Zm-ABP1 protein was purified (Lö bler and Klä mbt, 1985) and subsequently molecularly cloned Inohara et al, 1989;Tillmann et al, 1989). In the same year, it was also proven that ABP1 indeed binds auxin (Jones and Venis, 1989), a result that was later corroborated by the crystal structure determination of ABP1 cocrystallized with auxin (Woo et al 2002).…”

Auxin-binding protein located in the endoplasmic reticulum of maize shoots: molecular cloning and complete primary structure.

“…The ABP was found in the ER-rich fraction on sucrose density gradient centrifugation of tissue homogenates (18), and its distribution was distinguishable from those of marker enzymes of plasma membranes, tonoplasts, dictyosomes, mitochondria, and proplastids (38)(39)(40). The ABP was solubilized without using detergent (41), suggesting that it is not an integral membrane protein.…”

Membrane potential‐dependent calcium transport in right‐side‐out plasma membrane vesicles from Zea mays L. roots

“…Table 1 shows the activities and percentage recoveries of various marker enzymes in microsomes, the final plasma membrane-enriched fraction (U, ) and the contaminant-enriched lower phase from the first partition (L1). The plasma membrane-specific markers, vanadatesensitive ATPase (O'Neill and Spanswick, 1984) and glucan synthetase II (Ray, 1977) were enriched in U2 relative to their activities in the microsomes.…”