The obligate intracellular parasitic bacteria rickettsiae are more closely related to mitochondria than any other microbes investigated to date. A rickettsial putative peptidase (RPP) was found to resemble the ␣ and ␤ subunits of mitochondrial processing peptidase (MPP), which cleaves the transport signal sequences of mitochondrial preproteins. RPP showed completely conserved zinc-binding and catalytic residues compared with ␤-MPP but barely contained any of the glycine-rich loop region characteristic of ␣-MPP. When the biochemical activity of RPP purified from a recombinant source was analyzed, RPP specifically hydrolyzed basic peptides and presequence peptides with frequent cleavage at their MPP-processing sites. Moreover, RPP appeared to activate yeast ␤-MPP so that it processed preproteins with shorter presequences. Thus, RPP behaves as a bifunctional protein that could act as a basic peptide peptidase and a somewhat regulatory protein for other protein activities in rickettsiae. These are the first biological and enzymological studies to report that a protein from a parasitic microorganism can cleave the signal sequences of proteins targeted to mitochondria.The endosymbiont hypothesis for the origin of mitochondria in eukaryotes is now widely accepted, accompanied by a growing interest in evolution as entire genomic sequences are revealed from various organisms. According to this hypothesis, a free-living bacterium as an organelle progenitor once entered an anaerobic organism, which is thought to be an archaebacterial ancestor, and established a constitutive endosymbiotic relationship with the host cells, mainly to supply ATP (7,11,17). Modern mitochondria originated when the parasitic invader lost most of its own genome and began to depend on nuclearly encoded proteins for its biogenesis, although it retained control of the eukaryotic cell viability via metabolic and apoptotic pathways.Most mitochondrial proteins are encoded in the nucleus and synthesized by cytoplasmic ribosomes as preproteins with Nterminal presequences that are required for targeting to mitochondria (3, 12). These preproteins are unfolded and imported into the mitochondrial matrix across the double membrane through protein translocation machinery comprising a translocase on the outer mitochondrial membrane and a translocase on the inner mitochondrial membrane (20,24,26,27). Finally, the presequences are cleaved by a matrix-located metalloendopeptidase, i.e., mitochondrial processing peptidase (MPP) (10, 13, 16). Overall, this proteolytic processing is involved in maturation of the mitochondrial proteins and is essential for eukaryotic cell viability from unicellular (30) to multicellular (22) organisms. Therefore, the ␣ and ␤ subunits of MPP (␣-and ␤-MPP, respectively) tightly regulate the protease action and specifically cleave the preproteins.The genome sequences of the obligate intracellular parasitic bacteria rickettsiae (the agents that cause typhus) reveal gene profiles strikingly similar to those of mitochondria (2,18,23). Among ...

We report here the isolation of 44 genes that are upregulated after serum starvation and/or contact inhibition. These genes have been termed TIGA, after Transcript Induced by Growth Arrest. We found that there are two kinds of G0 phases caused by serum starvation, namely, the shallow G0 (or G0/G1) and the deep G0 phases. The shallow G0 is induced by only a few hours of serum starvation, while deep G0 is generated after 3 days of serum starvation. We propose that mammalian cells enter deep G0 through a G0 gate, which is only opened on the third day of serum starvation. TIGA1, one of the uncharacterized TIGA genes, encodes a homolog of cyanate permease of bacteria and localizes in mitochondria. This suggests that Tiga1 is involved in the inorganic ion transport and metabolism needed to maintain the deep G0 phase. Ectopic expression of TIGA1 inhibited not only tumor cell proliferation but also anchorage-independent growth of cancer cell lines. A microsatellite marker, ENDL-1, allowed us to detect loss of heterozygosity around the TIGA1 gene region (5q21–22). Further analysis of the TIGA genes we have identified here may help us to better understand the mechanisms that regulate the G0 phase.