The catalytic domain of the vacuolar proton ATPase is composed of a hexamer of three A subunits and three B subunits. Here we describe the cloning and characterization of a cDNA isoform of subunit B, HO57, from an osteoclastoma cDNA library. HO57 is represented by three species of mRNA of 1.6, 2.6 and 2.8 kb and is expressed at low levels in a range of human tissues, but at significantly higher levels in brain, kidney and osteoclastoma, and is probably an ubiquitously expressed isoform. In contrast, the kidney-specific isoform has an mRNA of 2 kb and is specifically expressed at high levels only in kidney and, at a lower level, in placenta. Thus the HO57 isoform is integral to the vacuolar ATPase found in the general secretory system of all cells as well as in vacuolar-ATPase-rich sources such as neurones and osteoclasts, whereas both the kidney-specific isoform and HO57 are highly expressed in the kidney. Furthermore, we show by in situ hybridization that HO57 is the only isoform that is exclusively and highly expressed by osteoclasts.

We have identified a cDNA coding for a protein of 160 kDa which is expressed in in vitro cultured human peripheral blood monocytes. The predicted amino acid sequence contains an alpha‐helical rod domain possessing features characteristic of intermediate filament proteins. However, the immunocytochemical staining pattern, abundance and solubility in Triton X‐100/high salt buffers suggest that this protein is probably only associated with the intermediate filament network and represents a new type of intermediate filament associated protein. In a survey of normal, inflammatory and human tumour tissue samples, this protein, which we have named restin, was found to be highly expressed in Reed‐Sternberg cells, the tumoral cells diagnostic for Hodgkin's disease. We suggest that restin overexpression may be a contributing factor in the progression of Hodgkin's disease.

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