Autar K. Mattoo

Identification of a primary in vivo degradation product of the rapidly-turning-over 32 kd protein of photosystem II

The 32 kd photosystem II protein of plant chloroplasts is rapidly turned over in the light. The initial events in the degradation of the 32 kd protein were studied. A 23.5 kd breakdown product was identified in Spirodela oligorrhiza membranes using immunological analysis. The 23.5 kd polypeptide was shown to be derived from the amino‐terminal portion of the 32 kd protein using partial proteolytic fingerprinting. An in vivo precursor–product relationship between the 32 kd protein and the 23.5 kd polypeptide was kinetically demonstrated by radiolabeling and pulse‐chase experiments. The cleavage site yielding the 23.5 kd polypeptide was localized to a functionally active region (between helices IV and V) of the 32 kd protein. We propose that an alpha‐helix‐destabilizing ‘degradation’ sequence, bordered by arginine residues 225 and 238, is involved in the formation of the 23.5 kd polypeptide.

Temperature-dependent inhibitory effects of calcium and spermine on ethylene biosynthesis in apple discs correlate with changes in microsomal membrane microviscosity☆

Abstract Inhibition of ethylene biosynthesis in apple fruit discs by calcium and spermine differed in character in at least three respects: (i) as the fruit ripened inhibition due to spermine decreased whereas that by calcium increased; (ii) inhibition by calcium was transitory, whereas that by spermine was persistent; (iii) at temperatures below 12°C calcium inhibited more than spermine whereas above 12°C the reverse was true. Temperature-dependence of the spermine and calcium effect on ethylene biosynthesis was correlated with specific changes induced by them in the microviscosity of microsomal membranes of apple tissue. Pretreatment with calcium resulted in lower microviscosity of the membranes compared to those of buffer-treated discs and in both cases microviscosity increased with increase in temperature of incubating solution containing fruit discs. Spermine stabilized membrane fluidity in apple discs incubated at temperatures above 4°C, thereby preventing the natural increase in microviscosity which occurred with increasing incubation temperature. These effects of calcium and spermine on membrane microviscosity required preincubation of fruit discs with the test compound since addition of spermine or calcium directly to microsomal membranes immediately before assay had no effect on their microviscosity.

Membrane association and some characteristics of the ethylene forming enzyme from etiolated pea seedlings.

Summary When freshly prepared homogenates of etiolated pea ( Pisum sativum L. cv Calvedon) subhook segments were fractionated by high speed centrifugation, the enzyme catalyzing the conversion of 1-aminocyclopropane-l-carboxylic acid (ACC) to ethylene was found associated with the particulate fraction. However, on aging the homogenates at 5°C prior to fractionation, 50 to 75% of the enzyme activity partitioned into the soluble fraction; this solubilization led to a highly activated enzyme form. Both the particulate and soluble enzyme exhibited non-linear substrate saturation kinetics and were inhibited to similar extents by ascorbic acid, EDTA, CoCl 2 and limiting oxygen. However, they differed in their response to incubation with n-propylgallate, dithiothreitol, CaCl 2 and 100% oxygen. Calcium stimulated only the particulate form and increased both the ‘low K m ’ for ACC from 2.99 to 5.58 mM and apparent Vmax from 88 to 285 nl/mg protein/h.

Presence of the rapidly-labelled 32 000-dalton chloroplast membrane protein in triazine resistant biotypes

Introduction A large number of herbicides, including triazines, inhibit photosynthesis by blocking electron transport at the reducing side of photosystem II [l-3]. The triazines are presently considered to interfere with the electron flow between Q, the primary electron accep- tor of photosystem II, and the secondary quencher, B. Binding studies with radiolabelled atrazine

Redox-Regulated Protein Phosphorylation and Photosystem II Function

Oxygenic photosynthetic organisms derive electrons from H2O and evolve molecular oxygen as a by-product of photosynthesis. Such organisms include higher plants, algae, and cyanobacteria. Oxygenic photosynthesis is mediated by two membrane-bound, pigment-protein complexes: Photosystem I (PSI) and Photosystem II (PSII). PSII carries out the reactions resulting in the oxidation of water and the reduction of plastoquinone while PSI utilizes reduced plastoquinone to drive the reduction of NADP. The reaction center of a photosystem is defined as the minimal unit necessary for primary charge separation and stabilization. The solution of the x-ray crystal structure of the anoxygenic bacterial photosynthetic reaction center (Deisenhofer et al., 1985; Allen et al., 1987) led to the prediction (Trebst, 1986) and subsequent verification (Nanba and Satoh, 1987; Marder et al., 1987) that the PSII reaction center consists of a heterodimer of homologous proteins, D1 and D2, and cytochrome b559. The D1/D2 heterodimer is thought to contain all the electron carriers and cofactors necessary for electron transport through the reaction center: P680; the Yz and YD electron donor tyrosines on D1 and D2, respectively; pheophytin; the plastoquinone electron acceptors QA and QB; and non-heme iron (for reviews see Rutherford, 1989; and Mattoo et al., 1989).