We have determined the cause of an unusual Cl inhibitor abnormality in a large kindred. We previously found that half of serum Cl inhibitor molecules in affected kindred members are normal. The other half complexed with Cls but showed little complex formation with Clr. These molecules also appeared to be relatively resistant to digestion by trypsin. Taken together, the findings suggested that members of this kindred are heterozygous for an unusual Cl inhibitor mutation. Sequencing of genomic DNA from the kindred revealed that thymine has replaced cytosine in the codon for Ala443 (P2 residue) in one Cl inhibitor allele, resulting in substitution with a Val residue. To test the effect of this substitution, a mutant Cl inhibitor containing Ala"3--Val was constructed by site-directed mutagenesis and expressed in COS-1 cells. Both the Ala'3--Val mutant and the wild-type Cl inhibitor complexed completely with Cls, kallikrein, and coagulation Factor XIIa after incubation at 37°C for 60 min. In contrast, the mutant inhibitor failed to complex completely with Clr under the same conditions. Time course analysis showed that the ability of the mutant to complex with Cls is also impaired: although it complexed completely in 60 min, the rate of complex formation during a 0-60-min incubation was decreased compared with wild-type Cl inhibitor. The mutant inhibitor also formed a complex with trypsin, a serine protease that cleaves, and is not inhibited by, wild-type Cl inhibitor. The Ala"3-Val mutation therefore converts Cl inhibitor from a substrate to an inhibitor of trypsin. These studies emphasize the role of the P2 residue in the determination of target protease specificity. (J. Clin. Invest. 1995. 95:1299-1305

CTP synthase (CTPS) has been demonstrated to form evolutionarily-conserved filamentous structures termed cytoophidia whose exact cellular functions remain unclear, but they may play a role in intracellular compartmentalization. We have previously shown that the mammalian target of rapamycin complex 1 (mTORC1)–S6K1 pathway mediates cytoophidium assembly in mammalian cells. Here, using the fission yeast Schizosaccharomyces pombe as a model of a unicellular eukaryote, we demonstrate that the target of rapamycin (TOR)-signaling pathway regulates cytoophidium formation (from the S. pombe CTPS ortholog Cts1) also in S. pombe. Conducting a systematic analysis of all viable single TOR subunit–knockout mutants and of several major downstream effector proteins, we found that Cts1 cytoophidia are significantly shortened and often dissociate when TOR is defective. We also found that the activities of the downstream effector kinases of the TORC1 pathway, Sck1, Sck2, and Psk1 S6, as well as of the S6K/AGC kinase Gad8, the major downstream effector kinase of the TORC2 pathway, are necessary for proper cytoophidium filament formation. Interestingly, we observed that the Crf1 transcriptional corepressor for ribosomal genes is a strong effector of Cts1 filamentation. Our findings connect TOR signaling, a major pathway required for cell growth, with the compartmentalization of the essential nucleotide synthesis enzyme CTPS, and we uncover differences in the regulation of its filamentation among higher multicellular and unicellular eukaryotic systems.

DNA damage response and repair proteins are centrally involved in genome maintenance pathways. Yet, little is known about their functional role under non-DNA damage-inducing conditions. Here we show that Rad9 checkpoint protein, known to mediate the damage signal from upstream to downstream essential kinases, interacts with Aft1 transcription factor in the budding yeast. Aft1 regulates iron homeostasis and is also involved in genome integrity having additional iron-independent functions. Using genome-wide expression and chromatin immunoprecipitation approaches, we found Rad9 to be recruited to 16% of the yeast genes, often related to cellular growth and metabolism, while affecting the transcription of ∼2% of the coding genome in the absence of exogenously induced DNA damage. Importantly, Rad9 is recruited to fragile genomic regions (transcriptionally active, GC rich, centromeres, meiotic recombination hotspots and retrotransposons) non-randomly and in an Aft1-dependent manner. Further analyses revealed substantial genome-wide parallels between Rad9 binding patterns to the genome and major activating histone marks, such as H3K36me, H3K79me and H3K4me. Thus, our findings suggest that Rad9 functions together with Aft1 on DNA damage-prone chromatin to facilitate genome surveillance, thereby ensuring rapid and effective response to possible DNA damage events.

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