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(Received for publication, January 25, 1996, and in revised form, April 17, 1996)
From the Department of Microbiology, Michigan State University,
East Lansing, Michigan 48824
Poly(3-hydroxybutyrate) (PHB) is well-known as a
high molecular weight homopolymer of R-3-hydroxybutyrate
which accumulates in storage granules within the cytosols of certain
bacteria. Escherichia coli does not amass these granules;
however, small amounts of low molecular weight PHB (<0.02% of dry
weight) have been found in the plasma membranes in complexes with
calcium polyphosphate; the complexes serve as voltage-activated calcium
channels. Here we report that polyphosphate-complexed PHB is only a
minor fraction of the polyester in E. coli. PHB comprises
0.36 to 0.55% of the dry weight of log-phase cells, depending on
culture medium, and this amount increases by 15 to 20% when the cells
are made genetically competent. The PHB is widely distributed
throughout the cell, wherein it is primarily associated with proteins.
The identity of protein-associated PHB was established by antibody
reaction, chemical assay, and 1H NMR spectroscopy. As
expected, the physical and chemical properties of protein-associated
PHB were found to be considerably different from those of the bulk
polymer or granule PHB, e.g. protein-PHB complexes are
normally insoluble in chloroform, soluble in water and alkaline
hypochlorite, and are converted to crotonic acid more slowly on heating
in concentrated sulfuric acid. Our studies indicate that the majority
of cellular PHB (over 80%) is located in cytoplasmic proteins,
especially proteins of the ribosomal fraction. Western immunoblots,
probed with polyclonal anti-PHB IgG, revealed a number of
PHB-polypeptides having a wide range of molecular weights in all cell
fractions. These results suggest that PHB is a fundamental constituent
of cells that may have physiological functions in addition to
facilitating ion transmembrane transport or serving as a carbon
reserve.
Volume 271, Number 36,
Issue of September 6, 1996
pp. 22196-22202
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.
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