The smaller protein formed as a ubiquitin fusion in Drosophila is processed from ubiquitin and found on the 60S ribosomal subunit.

The only gene in Drosophila melanogaster for a 52 amino acid ribosomal protein (CEP52) is fused to a ubiquitin coding sequence. This study examines expression and proteolytic processing of the encoded fusion protein. Most antibody preparations made against a portion of human CEP52 readily detect the insect protein. The size of the immunoreactive polypeptide indicates that CEP52 is cleaved from ubiquitin and this apparent proteolytic processing was confirmed by amino-terminal sequence analysis of CEP52 isolated by two-dimensional gel electrophoresis. Ribosomes from embryonic, larval and adult Drosophila melanogaster contain equivalent amounts of CEP52 and the protein is associated with the large ribosomal subunit. Stained two-dimensional gels indicate that the quantity of CEP52 associated with ribosomes is similar to that of other ribosomal proteins of corresponding size. A previous investigation had indicated the possibility of intact ubiquitin-CEP52 fusion protein in Dictyostelium discoideum, Saccharomyces cerevisiae and Drosophila melanogaster. One of three antibody preparations used in this study of insect CEP52 reacts with a 40S subunit protein that is the correct size to be the uncleaved fusion protein. However, the putative fusion protein does not react with ubiquitin antibodies and has negligible positive charge at pH5, demonstrating that it is not unprocessed ubiquitin-CEP52.