Mycobacterium bovis Bacillus Calmette-Guerin and its cell wall complex induce a novel lysosomal membrane protein, SIMPLE, that bridges the missing link between lipopolysaccharide and p53-inducible gene, LITAF(PIG7), and estrogen-inducible gene, EET-1.

LITAF and PIG7 encode an identical protein, and they have recently been reported as lipopolysaccharide and p53-inducible genes, respectively. By using the differential display approach, we identified a Mycobacterium bovis BCG cell wall skeleton (BCG-CWS)-inducible gene fragment from human monocytes, showing no homology to any reported gene. Full-length cloning of this fragment reveals the following. 1) The differential display product represents the incomplete 3'-untranslated region of LITAF/PIG7. 2) The coding region of the transcript differs from LITAF/PIG7 due to an absence of a single guanine residue, resulting in a potential translational frameshift. 3) The newly coded protein turns out to be 86% identical and 90% similar to an estrogen-inducible rat gene, EET-1. Repeated analysis, expressed sequence tag search, comparison with homologues, and genome sequence analysis confirmed the absence of the single guanine residue. One interesting feature of this protein is that it possesses the RING domain signature and is predicted to be localized in the nucleus. However, detailed analysis together with experimental evidence suggests it is neither a RING family member nor a nuclear protein. Comparison of a total collection of 18 proteins from various species indicates that proteins of this family are small in size and mainly conserved at the C-terminal domain with a unique motif. We characterize this novel protein as an unglycosylated small integral membrane protein of the lysosome/late endosome (SIMPLE) whose expression is elicited in monocytes by live and heat-killed BCG, BCG cell wall complex, lipopolysaccharide, and tumor necrosis factor-alpha. To our knowledge this is the first report of pathogen-associated molecular pattern (PAMP)-induced differential expression of a lysosomal membrane protein presumably involved in apoptosis.