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.
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