Fractionation by Ultracentrifugation of Gram Negative Cytoplasmic and Membrane Proteins Protein fractionation is a useful separation process which divides membrane proteins (including those located in the outer and inner membrane) and cytoplasmic proteins into discrete fractions. Fractionation of proteins can simplify analysis of the numbers of proteins present, and therefore make easier to characterize any environmentally or mutation induced changes in expression profiles, or changes in protein strucutre resulting from post-translational modification. This protocol is derived from Haigh et al. (2013) and it is specific to Gram negative bacteria.