Piggy-backing the bacterial chromosome: positioning of protein complexes by chromosome segregation

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When a cell divides, in general it produces a reasonable replica of itself. As the cell grows it makes new proteins, so that, before division it is reasonable to assume it has twice as many proteins as the initial cell. When that cell divides each daughter needs a chromosome to make those proteins, but are the proteins just a free soup that gets randomly divided between the daughter cells? That may be the case for some proteins, but other proteins need to be divided with partner proteins in the right ratio to work. This can be done by making tight, stable complexes that get divided together, but other groups of proteins need mechanisms that ensure each daughter cell has the correct proteins in the correct ratio. Membranes have long been know to be provide a framework for allowing proteins to come together, organising receptor proteins that respond to changes in the external world into interacting protein complexes. What about proteins that are soluble and do not respond to the external environment-how to they organise and how do cells ensure daughter cells inherit the right number of complexes. It has recently been realised that in bacterial cells some protein complexes can "piggy-back" on the duplicated and segregating chromosomes, with a protein loosely covering the chromosome surface and also interacting with a complex of proteins. As the chromosome segregates this protein carries this large protein "cargo" to the daughter cell. The protein which loosely associates with the chromosome is related to proteins known to also ensure daughter cells inherit the right genetic material and it is now clear that bacteria use this system to segregate a wide range of protein complexes needed in the correct ratio to function properly. As this is a common system across bacteria, it could be harnessed to organise other protein complexes between bacteria. In many metabolic pathways efficient rates are achieved when proteins are closely associate ensuring local high concentrations and fast substrate transfer. If these pathways are expressed in alien systems the proteins are often diffuse and disorganised, resulting in inefficient rates of activity. In addition many proteins function with partner proteins and to understand how they work ideally their structures need to be characterised when bound, but usually the proteins are purified individually and mixed or when expressed individually they are insoluble. It is possible we can use this natural system for segregating protein complexes to keep other proteins in functional complexes and at high local concentrations. Using this reduced system we could either use the system in synthetic biology for allowing a daughter cells to segregate proteins in functional but high ratios for a pathway to efficiently produce a desired product or it will allow expression of protein partners in an environment that will allow their isolation and structural analysis. To be able to harness this newly identified mechanism we need to understand the essential components holding the proteins together as complexes, causing the protein complexes to duplicate and the association with the chromosome on the chromosome associated protein.