Cell: Researchers Found a Novel Rule of Membrane Proteins
Like the teeth of a zipper, the charged amino acids (as the red and blue color show) form connections between protein segments and thus form pores in the cell membrane.
Membrane proteins, attached to or associated with the membrane of a cell or an organelle, perform a variety of functions vital to the survival of organisms. They are regarded as the "molecular machines" in biological cell envelopes. They manage various processes, such as the transport of molecules across the lipid membrane, signal transduction. Their shape, for instance, folding of the molecules, plays a decisive role in the formation of, e.g., pores in the cell membrane.
In the recently Cell magazine, researchers represented a novel charge zipper principle used by proteins to form functional units. This novel principle does not only seem to play a role in protein transport, but also in the attack of certain antimicrobial peptides on bacteria, or in their formation of biofilms as a response to stress.
In the study, researchers investigate the Twin-arginine translocase (Tat). Tat is used in the cell membrane of bacteria as an export machinery for folded proteins. Several TatA subunits assemble as a pore that can adapt its diameter to the size of the cargo to be transported.
But how can such a pore be built up from TatA proteins? How can they reversibly form a huge hole in the membrane for a variety of molecules to pass through without causing leakage of the cell? In order to figure out these questions, the researchers studied the molecular structure of TatA protein from the bacterium B. subtilis.
Subsequently researchers found that the bacterium folds into a rather rigid, rod-shaped helix that is followed by a flexible, extended stretch. Many amino acids in the helix and the adjacent stretch carry positive or negative charges. To their surprise, the sequence of charges on the helix is complementary to those in the adjacent stretch of the protein. When the protein is folded up at the connection point like a pocket knife, positive and negative charges will always meet and attract each other. Hence, the protein links up both of its segments, just like the interlocking teeth of a zipper.
Moreover, the structure also interacts with the nearby proteins: in addition to folding up, every TatA protein also forms charge zippers with both of its neighbors. Researchers therefore gave computer simulations which resulted in stable and flexible connections between the adjacent molecules. In this way, any number of proteins can be linked together to form an uncharged ring, which thus lines the TatA pore in the hydrophobic membrane.
This novel rule is important in protein transport as well as certain antimicrobial peptides and the generation of biofilm during physical response.
Tags: Membrane Protein, Protein Segment, Protein Connection, Tat, Subtilis