Immunolocalization of a specific type of prosome close to the bile canaliculi in fetal and adult rat liver

Prosomes are mRNA-associated RNP particles and cofactors of untranslated (ribosome-) free mRNP having a multicatalytic proteinase (MCP; proteasome) activity. The expression of prosomal proteins in fetal development of the rat liver was investigated by indirect immunofluorescence, using a panel of monoclonal antibodies to individual prosomal proteins (p-mAbs). In all fetal and adult stages tested, strong immunofluorescence staining was observed with the p31K-specific p-mAb exclusively, whilst Western blot analysis showed reactivity also with the p27K and p33K antigens. Double labeling with the 31K p-mAb and an anti-cytokeratin antibody showed that the prosome antigen superimposes partially onto this type of intermediate filaments (IF), confirming earlier observations made on cultured cell lines of various types. Most interestingly, the p31K antigen was found preferentially in the pericanalicular zone of hepatocytes in the developing liver, from day 17 onwards up to the adult state. This shows a preferential concentration of prosomes of a specific type, including the p31K antigen, in the morphologically and possibly functionally specialized apical domain of the hepatocyte, in a differentiation-related fashion.