New insights into bacterial protein secretion

A biofilm is a bacterial community that is surrounded by a protective capsule consisting of sugar polymers and ‘curli’. Using X-ray crystallography, Parveen Goyal and Han Remaut (VIB Structural Biology Research Center, Vrije Universiteit Brussel) created a detailed three-dimensional image of the pores through which the curli building blocks cross the bacterial cell envelope.

What was the most challenging aspect in the process of unraveling the structure?
Parveen: Solving X-ray structures of membraneembedded proteins can be a daunting task. It took a lengthy search for conditions that produced crystals with a good enough diffraction quality. Then began the task of translating that structural data into new biological insights into the secretion system.

Why is it important to know the structure of these pores?
Parveen: Transporting the curli building blocks to the outside is a remarkable feature of the bacteria. It requires energy and as curli-forming bacteria have a double-layered cell envelope, they do not have access to the usual cellular motors that drive protein transport channels. With the structure of the secretion channel as our guide, we were for the first time able to build a model of how bacteria can overcome this barrier.

Han: By determining the three-dimensional structure, it is now possible to develop small molecules that fit like a stopper in the pore of the transporter. This inhibits the excretion of the curli building blocks and can prevent the formation of curli fibers and unwanted biofilms during infections or in industrial installations.

Are biofilms always a ‘bad thing’?
Han: Biofilms of non-pathogenic bacteria can also exhibit desirable properties that could be used for new applications. Because they are able to polymerize autonomously and are extremely sturdy, curli fibers have great potential as building blocks for functionalized nanowires. Curli fibers could be used as carriers for other proteins by using them to form so-called functional biofilms. For example, by coating the curli fibers with proteins and enzymes that absorb, convert or break down specific substances, they could be used for waste water treatment or the conversion of biofuels. This requires further research, but we already reported the first steps in this direction earlier this year.

Curli are amyloid fibers, a type of fiber until recently only known for its involvement in Alzheimer’s and Parkinson’s. Can this new knowledge lead to insights into those diseases?
Han: Amyloids are mostly known from proteinmisfolding events, and in particular the cytotoxic intermediates often associated with them. As we gain more insight into how bacteria are able to safely handle this amyloid formation, it will be interesting to see whether this will generate ideas for ways to prevent pathological amyloidosis.