The S-layer of Lampropedia hyalina is made of two separate layers. The inner, perforate layer, is composed of a 32kD polypeptide arranged in p6 symmetry with a lattice spacing of 14.6nm. The outer, punctate layer, is composed of tubular units connected by fine linking arms centred on p3 symmetry axes, to created a layer with p6 symmetry and a lattice constant of 25.6nm. Incubation of cell envelopes with 3M urea dissolved the punctuate layer and released three polypeptides of 60kD, 66kD and 240kD. These polypeptides reassembled to form the native punctate layer when dialysed against buffer containing 10mM CaCl{dollar}\sb2{dollar} or 10mM SrCl{dollar}\sb2{dollar}. The urea soluble polypeptides were fractionated by hydroxylapatite column chromatography. The isolated 60kD polypeptide formed ring-like structures, while the isolated 240kD polypeptide was visualized in negative stain as long, slightly curved threads. Fractions containing both the 60kD and 240kD polypeptides also contained complete assemblies of the punctate layer. This suggests that the central tubular units are composed of the 60kD polypeptide, while the linking arms consist of the 240kD polypeptide. One other fraction (66kD) may mediate attachment of the punctate layer to the underlying perforate layer thus forming the whole complex S-layer from at least four distinct polypeptides
