Topography and functional information of plasma membrane |
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Authors: | DeLan Sun JianMin Chen YanMei Song ChuanFeng Zhu GeBo Pan LiJun Wan |
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Institution: | (1) Key Laboratory of Photosynthesis and Environmental Molecular Physiology, Institute of Botany, Chinese Academy of Sciences, Beijing, 100093, China;(2) Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, China |
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Abstract: | By using atomic force microscope (AFM), the topography and function of the plasmalemma surface of the isolated protoplasts
from winter wheat mesophyll cells were observed, and compared with dead protoplasts induced by dehydrating stress. The observational
results revealed that the plasma membrane of living protoplasts was in a state of polarization. Lipid layers of different
cells and membrane areas exhibited distinct active states. The surfaces of plasma membranes were unequal, and were characterized
of regionalisation. In addition, lattice structures were visualized in some regions of the membrane surface. These typical
structures were assumed to be lipid molecular complexes, which were measured to be 15.8±0.09 nm in diameter and 1.9±0.3 nm
in height. Both two-dimensional and three-dimensional imaging showed that the plasmalemma surfaces of winter wheat protoplasts
were covered with numerous protruding particles. In order to determine the chemical nature of the protruding particles, living
protoplasts were treated by proteolytic enzyme. Under the effect of enzyme, large particles became relatively looser, resulting
that their width was increased and their height decreased. The results demonstrated that these particles were likely to be
of protein nature. These protein particles at plasmalemma surface were different in size and unequal in distribution. The
diameter of large protein particles ranged from 200 to 440 nm, with a central micropore, and the apparent height of them was
found to vary from 12 to 40 nm. The diameter of mid-sized protein particles was between 40–60 nm, and a range of 1.8–5 nm
was given for the apparent height of them. As for small protein particles, obtained values were 12–40 nm for their diameter
and 0.7–2.2 nm for height. Some invaginated pits were also observed at the plasma membrane. They were formed by the endocytosis
of protoplast. Distribution density of them at plasmalemma was about 16 pits per 15 μm2. According to their size, we classified the invaginated pits into two types-larger pits measuring 139 nm in diameter and
7.2 nm in depth, and smaller pits measuring 96 nm in diameter and 2.3 nm in depth. On dehydration-induced dead protoplasts,
the degree of polarization of plasma membranes decreased. Lipid molecular layers appeared relatively smooth, and the quantity
of integral proteins reduced a lot. Invaginated pits were still detectable at the membrane surface, but due to dehydration-induced
protoplast contraction, the orifice diameter of pits reduced, and their depth increased. Larger pits averagely measuring 47.4
nm in diameter and 31.9 nm in depth, and smaller pits measuring 26.5 nm in diameter and 43 nm in depth at average. The measured
thickness of plasma membranes of mesophyll cells from winter wheat examined by AFM was 6.6–9.8 nm, thicker in regions covered
with proteins. |
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Keywords: | atomic force microscope winter wheat protoplast plasma membrane surface membrane protein invaginated pits |
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