Cells are crowded with macromolecules, which restricts
the diffusion of proteins, especially in prokaryotic cells without active
transport in the cytoplasm. While examining the relationship between crowding,
ionic strength and protein diffusion, University of Groningen biochemists made
a fascinating discovery: positively charged proteins stick to the surface of
ribosome complexes. This explains why most water-soluble proteins carry an
overall negative charge. The speed of movement of proteins inside cells is important;
many procedures in biological cells depend on interactions between
macromolecules (proteins and nucleic acids) and so on their ability to find
each other. One of the professor from the University of Groningen stated that
the cell cytoplasm is an active place and this will affect protein and RNA diffusion.
His team observed the effects of flocking on diffusion, and found a correlation
between protein size and diffusion speed. The team then used three different
prokaryotes with increasing ionic strength: “the Gram-negative bacterium
Escherichia coli, the Gram-positive Lacto coccus lactis and the extremophile
Haloferax volcanii, which lives at very high salt concentrations” (University
of Groningen).
Different variants of Green Fluorescent Protein (GFP)
with surface charges were constructed, and then the movement of these GFP
variants in the three different cell types. They observed that positively
charged proteins would diffuse very slowly then, as the further studies were
going on it showed that the positive proteins did not bind to the DNA or the
cell membrane but to the ribosome complex. The new and unexpected insight that
protein movement is a function of protein charge may explain why it is hard to
express some proteins in bacterial systems with low ionic strength. They concluded that a higher ionic strength
reduces the stickiness of positively charged proteins and that could be a
valuable insight for the construction of protein expression platforms. A final is
that the genomes of several endosymbionts show an abundance of positively
charged proteins. They said that they have no explanation of how these
organisms are able to deal with slow diffusion and ribosomes being engulfed
with positive proteins.
References:
University of Groningen. (2017, December 6). How
ribosomes shape the proteome. ScienceDaily. Retrieved December 8, 2017
from www.sciencedaily.com/releases/2017/12/171206100101.htm
How Ribosomes Shape the Proteom. (2017, December 6).
Retrieved December 08, 2017, from http://www.sciencenewsline.com/news/2017120614520040.html
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