Orthogonal arrays of particles in non-pigmented cells of rat ciliary epithelium: relation to distribution of filipin- and digitonin-induced alterations of the basolateral membrane.

It has been suggested that orthogonal arrays of particles may increase the rigidity of plasma membrane, as does cholesterol. Therefore, using freeze-fractured non-pigmented ciliary epithelium, the distribution of such arrays was compared to the distribution of membrane deformations induced by the sterol-probes filipin and digitonin in different domains of the basolateral plasma membrane. The distribution of orthogonal arrays of particles was homogeneous between different regions of the basolateral membrane of the non-pigmented ciliary epithelium, while the number of filipin-induced alterations was nearly 4 times higher in the membrane domains not in contact with the basal lamina than in domains in contact with it. Contrary to the homogeneous distribution of arrays, digitonin-induced deformations also differed markedly in these two basolateral membrane domains. Considering that a marked positive response to sterol probes implies a high sterol content, we conclude that orthogonal arrays of particles can occur in plasma membrane regions well-provided with cholesterol and not in direct contact with the basal lamina. Other possible roles of these arrays are discussed.

A new pentraxin (serum amyloid P-component) in the rat: evidence for two quaternary structures and effect of ligands on self-association.

A new rat serum protein has been isolated by affinity chromatography using ethanolamine- or phosphoethanolamine-substituted agarose gels. This protein has the morphological and functional characteristics of serum amyloid P-component and C-reactive protein. It comprises C5 cyclic symmetry structure with non covalently cross-linked subunits which have calcium-dependent binding sites. We have called this protein rat serum amyloid P-component since it has all the properties typical of human serum amyloid P-component: it is made up to 10 subunits, it contains sialic acid and hexoses, it forms macroscopic polymers and its does not precipitate with pneumococcal C-polysaccharide. Rat amyloid P-component has three remarkable properties. Electron microscopy has shown that apart from pentagonal figures and stacked discs, rat P-component has a C10 cyclic symmetry structure. Rat amyloid P-component has an affinity for specific ligands, such as phosphorylcholine or phosphoethanolamine. These ligands are able to depolymerize self-associated rat P-component. With these characteristics, rat serum amyloid P-component could prove to be an important model in the study of the relations between amyloid P-component and amyloidosis.