Colchicine-binding protein from phloem and xylem of a higher plant

Binding of colchicine to a soluble fraction of carrot cells grown in suspension culture

The binding of [(3)H]colchicine to soluble component prepared from carrot (Daucus carota L.) cells in suspension culture was assayed by the diethylaminoethyl(DEAE)-cellulose powder method. The binding activity was very labile and the time course of the binding indicated that the colchicine-bound complex was also unstable. The reaction was enhanced by vinblastine but lumicolchicine had no effect. The optimum temperature for the reaction was 30° C, and the colchicine binding constant was calculated to be 3.5·10(4) l mol(-1) at 30° C.

Heterogeneity in phloem protein complements from different species : Consequences of hypotheses concerned with P-protein function

Protein subunits present in phloem exudate from 17 cultivars, 5 species and 3 genera of the Cucurbitaceae have been fractionated by SDS-gel electrophoresis. The degree of difference in the phloem protein patterns appears to reflect the taxonomic relationships of the plants: there were major differences among genera, significant differences and similarities among species, and relatively few differences among cultivars of a single species. A representative of another family (Acer pseudoplatanus; Aceraceae) provided a completely different electrophoretic pattern. Each plant displayed a consistent protein pattern, irrespective of the organ from which exudate was obtained or the age and physiological status of the plant. Neither complete etiolation nor transition to the flowering phase effected any change in phloem proteins. The differences in phloem proteins between plants and the unusual properties of major subunits from different protein complements, render improbable many of the present ideas concerning the origin and function of P-protein.

Is P-protein actin-like?-not yet

Microfilaments associated with cytoplasmic streaming in Nitella flexilis internodes can be decorated with heavy meromyosin (HMM) from rabbit, both in vitro in cytoplasmic suspensions, and in situ in glycerinated cell segments. The bound HMM consists of clearly discernible, polarized arrowheads in a regular repeat of 360-380 Å that are similar to those produced on F-actin. In contrast, similar arrowheads or decorations are not evident on P-protein filaments in sieve elements of glycerinated hypocotyl segments of Phaseolus vulgaris L. treated with HMM. Thus, these results contradict a recent claim that P-protein binds HMM and is actin-like. The mass of other evidence now available from diverse studies indicating that P-protein does not consist of actin or tubulin is discussed.

Studies on the possible occurrence of actomyosin-like proteins in phloem

Extracts of large quantities of petioles and of isolated vascular tissues of Heracleum mantegazzianum have been analysed for actomyosin-like contractile proteins. Concentrated preparations, subjected to a standard isolation and purification procedure for actomyosin, failed to demonstrate either superprecipitation or viscosity changes in response to the addition of ATP and divalent cations. In addition, ATPase activities in phloem and xylem extracts have been fractionated by gel electrophoresis and characterised with regard to their substrate specificity, pH optima and ion requirements. Phloem extracts provide two phosphatases: one is non-specific in its substrate requirements; the other is a nucleoside triphosphatase but is stimulated only by monovalent cations and is also present in xylem extracts. All the enzymes are strongly inhibited by divalent cations and do not possess any of the characteristics of ATPases associated with contractile systems. The results are discussed in relation to the postulated involvement of contractile proteins in the translocation of sugars in phloem.

Sieve-tube proteins from Cucurbita maxima

The two main proteins from the phloem exudate of Cucurbita maxima Duchesne have been isolated by ammonium-sulfate precipitation, DEAE-cellulose chromatography, and gel filtration, and have been characterized. They comprise about 40% each of the total protein. The amino-acid composition of these two proteins has been determined. Both are highly basic with an IEP above 9.5. The smaller protein has a molecular weight of ca. 30000 as determined by analytical ultracentrifugation, gel filtration, and SDS polyacrylamide gel electrophoresis. It easily dimerizes to a form which appears to be the naturally occurring structure. The larger protein has a molecular weight of 116000 (ultracentrifugation, SDS polyacrylamide gel electrophoresis). This protein forms a gel in the absence of SH-protecting agents. Several minor proteins have been detected by polyacrylamide gel electrophoresis.