Reference "Phosphorylation of serine-15 of maize leaf sucrose synthase. Occurrence in vivo and possible regulatory significance"

Reference ID

7368

Title

Phosphorylation of serine-15 of maize leaf sucrose synthase. Occurrence in vivo and possible regulatory significance

Source

Plant physiology, 1996, vol. 112, pp. 793-802

Authors (7)

Huber-S-C	Huber-J-L
Liao-P-C	Gage-D-A
Chourey-P-S	Hannah-L-C
Koch-K

Abstract

Experiments were conducted to determine whether sucrose synthase (SuSy) was
phosphorylated in the elongation zone of maize (Zea mays L.) leaves. The
approximately 90-kD subunit of SuSy was 32P-labeled on seryl residue(s) when
excised shoots were fed [32P]orthophosphate. Both isoforms of SuSy (the SS1 and
SS2 proteins) were phosphorylated in vivo, and tryptic peptide-mapping analysis
suggested a single, similar phosphorylation site in both proteins. A combination
of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry
and automated Edman sequencing analysis unequivocally identified the
phosphorylation site in the maize SS2 protein as serine-15. This site was
phosphorylated in vitro by endogenous protein kinase(s) in a strictly Ca(2+)-
dependent manner. A synthetic peptide, based on the phosphorylation site
sequence, was used to identify and partially purify an endogenous Ca(2+)-
dependent protein kinase(s) from the maize leaf elongation zone and expanding
spinach leaves. Phosphorylation of SuSy in vitro selectively activates the
cleavage reaction by increasing the apparent affinity of the enzyme for sucrose
and UDP, suggesting that phosphorylation may be of regulatory significance.
Conservation of the phosphorylation site, and the sequences surrounding it,
among plant species suggests that phosphorylation of SuSy may be widespread, if
not universal, in plants.