Molecular biotechnology now makes it possible to build elaborate systems models, but the systems biology community needs information standards if models are to be shared, evaluated and developed cooperatively.
We summarize the Systems Biology Markup Language (SBML) Level 1, a free, open, XML-based format for representing biochemical reaction networks. SBML is a software-independent language for describing models common to research in many areas of computational biology, including cell signaling pathways, metabolic pathways, gene regulation, and others.
The specification of SBML Level 1 is freely available from http://www.sbml.org/Read more
Extracellular levels of the excitatory neurotransmitter glutamate in the nervous system are maintained by transporters that actively remove glutamate from the extracellular space. Homozygous mice deficient in GLT-1, a widely distributed astrocytic glutamate transporter, show lethal spontaneous seizures and increased susceptibility to acute cortical injury. These effects can be attributed to elevated levels of residual glutamate in the brains of these mice.Read more
X chromosome-linked inhibitor of apoptosis (XIAP) is an endogenous inhibitor of caspase-3, -7, and -9. Smac/DIABLO, an inhibitor of XIAP, is released from mitochondria upon receiving apoptotic stimuli and binds to the BIR2 and BIR3 domains of XIAP, thereby inhibiting its caspase-inhibitory activity. Here we report that a serine protease called HtrA2/Omi is released from mitochondria and inhibits the function of XIAP by direct binding in a similar way to Smac. Moreover, when overexpressed extramitochondrially, HtrA2 induces atypical cell death, which is neither accompanied by a significant increase in caspase activity nor inhibited by caspase inhibitors, including XIAP. A catalytically inactive mutant of HtrA2, however, does not induce cell death. In short, HtrA2 is a Smac-like inhibitor of IAP activity with a serine protease-dependent cell death-inducing activity.Read more
We have shown previously that chromosome VI of Saccharomyces cerevisiae contains nine origins of DNA replication that differ in initiation frequency and replicate sequentially during the S phase of the cell cycle. Here we show that there are links between activation of these multiple origins and regulation of S-phase progression. We study the effects of a DNA-damaging agent, methyl methane sulphonate (MMS), and of mutations in checkpoint genes such as rad53 on the activity of origins, measured by two-dimensional gel analysis, and on cell-cycle progression, measured by fluorescence-activated cell sorting. We find that when MMS slows down S-phase progression it also selectively blocks initiation from late origins. A rad53 mutation enhances late and/or inefficient origins and releases the initiation block by MMS. Mutation of rad53 also results in a late origin becoming early replicating. We conclude that rad53 regulates the timing of initiation of replication from late origins during normal cell growth and blocks initiation from late origins in MMS-treated cells. rad53 is, therefore, involved in the cell's surveillance of S-phase progression. We also find that orc2, which encodes subunit 2 of the origin-recognition complex, is involved in suppression of late origins.Read more
Mammalian kinetochores contain the centromere-specific histone H3 variant CENP-A, whose incorporation into limited chromosomal regions may be important for centromere function and chromosome segregation during mitosis. However, regulation of CENP-A localization and its role have not been clear. Here we report that the fission yeast homolog SpCENP-A is essential for establishing centromere chromatin associated with equal chromosome segregation. SpCENP-A binding to the nonrepetitious inner centromeres depended on Mis6, an essential centromere connector protein acting during G1-S phase of the cell cycle. Mis6 is likely required for recruiting SpCENP-A to form proper connection of sister centromeres.Read more
The effects of various divalent cations in the external solution upon the Ca spike of the barnacle muscle fiber membrane were studied using intracellular recording and polarizing techniques. Analysis of the maximum rate of rise of the spike potential indicates that different species of divalent cations bind the same membrane sites competitively with different dissociation constants. The overshoot of the spike potential is determined by the density of Ca (Sr) ions in the membrane sites while the threshold membrane potential for spike initiation depends on the total density of divalent cations. The order of binding among different divalent and trivalent cations is the following: La+++, UO2++ > Zn++, Co++, Fe++ > Mn++ > Ni++ > Ca++ > Mg++, Sr++.Read more
Fission yeast centromeres vary in size but are organized in a similar fashion. Each consists of two distinct domains, namely, the approximately 15-kilobase (kb) central region (cnt+imr), containing chromosome-specific low copy number sequences, and 20- to 100-kb outer surrounding sequences (otr) with highly repetitive motifs common to all centromeres. The central region consists of an inner asymmetric sequence flanked by inverted repeats that exhibit strict identity with each other. Nucleotide changes in the left repeat are always accompanied with the same changes in the right. The chromatin structure of the central region is unusual. A nucleosomal nuclease digestion pattern formed on unstable plasmids but not on stable chromosome. DNase I hypersensitive sites correlate with the location of tRNA genes in the central region. Autonomously replicating sequences are also present in the central region. The behavior of truncated minichromosomes suggested that the central region is essential, but not sufficient, to confer transmission stability. A portion of the outer repetitive region is also required. A larger outer region is necessary to ensure correct meiotic behavior. Fluorescence in situ hybridization identified individual cens. In the interphase, they cluster near the nuclear periphery. The central sequence (cnt+imr) may play a role in positioning individual chromosomes within the nucleus, whereas the outer regions (otr) may interact with each other to form the higher-order complex structure.Read more
We have isolated a novel actin filament-binding protein, named afadin, localized at cadherin-based cell-cell adherens junctions (AJs) in various tissues and cell lines. Afadin has one PDZ domain, three proline-rich regions, and one actin filament-binding domain. We found here that afadin directly interacted with a family of the immunoglobulin superfamily, which was isolated originally as the poliovirus receptor-related protein (PRR) family consisting of PRR1 and -2, and has been identified recently to be the alphaherpes virus receptor. PRR has a COOH-terminal consensus motif to which the PDZ domain of afadin binds. PRR and afadin were colocalized at cadherin-based cell-cell AJs in various tissues and cell lines. In E-cadherin-expressing EL cells, PRR was recruited to cadherin-based cell-cell AJs through interaction with afadin. PRR showed Ca2+-independent cell-cell adhesion activity. These results indicate that PRR is a cell-cell adhesion molecule of the immunoglobulin superfamily which is recruited to cadherin-based cell-cell AJs through interaction with afadin. We rename PRR as nectin (taken from the Latin word "necto" meaning "to connect").Read more