In vivo crystal formation in Escherichia coli of an over-expressed soluble form of penicillin-binding protein 5

Accumulation of either native membrane-bound or soluble variants of PBP5 over-expressed in teh cytoplasm was investigated by electron microscopy of ultra-thin sections. One of the soluble forms of PBP5 (PBP5s353) formed well-ordered crystals inside the cells. Cells sectioned perpendicular to their long axis showed a diamond-shaped cyrstal whereas cells cut parallel to their long axis contained a long, narrow crystal. In both sectioning directions an ordered ultra-structure was visible as shown by optical diffraction. Computer processing was used to enhance the crystal images. From this the unit cell parameters were calculated as a = 7.6 nm, b = 4 nm, c = 4.2 nm, γ = 75°. The calculated unit-cell volume of 120 nm3 is large enough to contain one protein molecule.