We have previously shown that rough endoplasmic reticulum (RER) from liver cell lines can give intact and enriched secretory protein mRNA (See project #Z01CP05739-01 LMO). Such mRNA was used as part of an assay system to determine which, if any, 3' cDNA fragments cloned by differential display correspond to secreted proteins. Out of 20 differentially expressed cDNA fragments, one was found novel by database sequence analysis and corresponding to a secretory protein by RER fraction-extracted RNA Northern analysis. We used 5' rapid amplification of cDNA ends (RACE) polymerase chain reaction (PCR) to clone the entire 3.7 kb mRNA from placenta mRNA, which was the only healthy tissue that expressed this novel clone. This gene contains a leader peptide sequence, a proline repeat, and seven potential membrane-spanning domains, according to computer analysis. The other 19 differentially expressed clones contain six complete novel genes, a histone-related gene, a transcriptional factor, and nuclear cytosolic proteins which are negative for RER localization, according to the previously discussed assay. We are currently focusing on our putative cell surface protein clone for protein expression, antibody production and patient sera/tissue screening to determine its diagnostic/ prognostic/therapeutic significance.
