The structure of a periodic cell wall component (HPI-layer of Micrococcus radiodurans)

The hexagonally packed interlayer (HPI-layer) from Micrococcus radiodurans cell walls has been studied by electron microscopy and subsequent digital image processing. The most prominent feature in the average images is a "complex" shaped like a "toothed wheel", which is perforated by a central pore and interconnected by fine spokes. This basic structural element is tentatively interpreted to represent the bulk of HPI-layer protein, intercalated by the other constituents: lipids, carotenoids and carbohydrates. It is suggested that the "toothed wheel" structure is a quite common element of periodic bacterial surface layers and that the different spacings observed with various species are due to variable amounts of intercalating material.