Passing a camel through the eye of a needle?
A recent study led by Professor Tracy Palmer at the University of Dundee discovered a new way by which bacteria can insert proteins into membranes, using two different transport machines. Here she gives us the lowdown on what it’s all about…
All cells are surrounded by lipid membranes; they keep the insides in, and the outsides out, but sometimes bacteria need to bring a molecule from the environment into the cell, or vice versa. The membrane itself is pretty much impermeable, so the cell uses specialised proteins to make channels in the membrane so they can control what moves from one side to the other.
Membrane proteins are unusual as they are mainly hydrophobic (water-hating), unlike most other proteins, which are hydrophilic (water-loving). This is because there is no water in the cell membrane, it is mostly made up of fats. This poses an interesting problem: how do they make a hydrophobic protein in the watery cytoplasm and then insert it into a hydrophobic membrane?
If membrane proteins accumulate in the cytoplasm the cell will die and so, to prevent this, membrane proteins are usually threaded directly from the ribosome (where they are made) into the membrane. This is known as ‘co-translational translocation’, and it prevents the hydrophobic bits being exposed to the cytoplasm. The protein thread is received by a specialised protein transport machine called Sec (the general secretion system), which inserts it into the membrane, where it can fold into its active state.
However, while this approach works for most membrane proteins, we discovered an important exception; a Rieske protein in the soil bacterium Streptomyces coelicolor. Rieske proteins contain iron sulphur clusters and these help to transfer electrons during respiration or photosynthesis – they’re really important and they are found in almost all organisms.
The iron-sulphur clusters in these proteins are inserted into the protein in the cytoplasm, so they have to be exported in their active state. Because the protein is fully folded, it’s too big to be threaded into the membrane by the ribosome and Sec. You can pass a strand of cotton through the eye of a needle, but a full t-shirt? Not so much…
This is where a second pathway comes in. The Tat secretion pathway allows fully-folded proteins to cross the cell membrane, via a pore that can increase or decrease in size depending on the size of protein passing through. For a long time it was thought that proteins destined for the membrane were either secreted via the Sec or Tat pathway. Our work shows for the first time that the two pathways can work together.
It turns out that the Rieske proteins of S. coelicolor (and related bacteria) are much larger and more hydrophobic than expected and this affects how the proteins are inserted into the membrane. The first half of the protein uses Sec to insert it into the membrane, while the folded half containing the iron-sulphur cluster uses the Tat pathway. This is all rather unexpected, as the Sec machinery is usually attached to the ribosome to thread the protein into the membrane as it is being made!
Editors note: It’s hard to get your head around how two pathways might interact to insert a single protein into the membrane but this does throw up some interesting opportunities for protein engineering. Since Sec and Tat recognise specific signal sequences it will be interesting to see if artificial proteins can be made and targeted to these two pathways.
Keller R, de Keyzer J, Driessen AJ and Palmer T (2012). Co-operation between different targeting pathways during integration of a membrane protein. J Cell Biol. 199: 303-15