Identification of a truncated ratp28-related protein expressed in kidney

An RT-PCR based strategy to clone the membrane-associated steroid binding protein ratp28 additionally amplified a novel sequence-related PCR product termed HC5. The HC5 PCR product was cloned and sequenced and showed 94% nucleotide sequence similarity to ratp28. The HC5 cDNA sequence open reading frame encodes a predicted 75 amino acid (8.0kDa) protein, and is therefore truncated compared to ratp28 (195 amino acids, 21.6kDa). In vitro transcription and translation of the HC5 cDNA resulted in the production of 2 proteins of approximately 8 and 6kDa. Restriction digests from various tissues demonstrated that liver and heart expressed primarily ratp28 mRNA whereas kidney and blood contained both ratp28 and HC5 transcripts. Phage display was employed to generate an antibody fragment to a peptide sequence conserved in ratp28 and HC5. Western blotting identified a 10kDa protein in cytosolic fractions of rat kidney. The function of HC5 remains to be determined.