Background— Diminished cardiac S100A1 protein levels are characteristic of ischemic and dilated human cardiomyopathy. Because S100A1 has recently been identified as a Ca 2+ -dependent inotropic factor in the heart, this study sought to explore the pathophysiological relevance of S100A1 levels in development and progression of postischemic heart failure (HF). Methods and Results— S100A1-transgenic (STG) and S100A1-knockout (SKO) mice were subjected to myocardial infarction (MI) by surgical left anterior descending coronary artery ligation, and survival, cardiac function, and remodeling were compared with nontransgenic littermate control (NLC) and wild-type (WT) animals up to 4 weeks. Although MI size was similar in all groups, infarcted S100A1-deficient hearts (SKO-MI) responded with acute contractile decompensation and accelerated transition to HF, rapid onset of cardiac remodeling with augmented apoptosis, and excessive mortality. NLC/WT-MI mice, displaying a progressive decrease in cardiac S100A1 expression, showed a later onset of cardiac remodeling and progression to HF. Infarcted S100A1-overexpressing hearts (STG-MI), however, showed preserved global contractile performance, abrogated apoptosis, and prevention from cardiac hypertrophy and HF with superior survival compared with NLC/WT-MI and SKO-MI. Both Gq-protein–dependent signaling and protein kinase C activation resulted in decreased cardiac S100A1 mRNA and protein levels, whereas Gs-protein–related signaling exerted opposite effects on cardiac S100A1 abundance. Mechanistically, sarcoplasmic reticulum Ca 2+ cycling and β-adrenergic signaling were severely impaired in SKO-MI myocardium but preserved in STG-MI. Conclusions— Our novel proof-of-concept study provides evidence that downregulation of S100A1 protein critically contributes to contractile dysfunction of the diseased heart, which is potentially responsible for driving the progressive downhill clinical course of patients with HF.

Phosphorylation of histone H2A or H2AX is an early and sensitive marker of DNA damage in eukaryotic cells, although mutation of the conserved damage-dependent phosphorylation site is well tolerated. Here, we show that H2A phosphorylation is required for cell-cycle arrest in response to DNA damage at the G1/S transition in budding yeast. Furthermore, we show that the tandem BRCT domain of Rad9 interacts directly with phosphorylated H2A in vitro and that a rad9 point mutation that abolishes this interaction results in in vivo phenotypes that are similar to those caused by an H2A phosphorylation site mutation. Remarkably, similar checkpoint defects are also caused by a Rad9 Tudor domain mutation that impairs Rad9 chromatin association already in undamaged cells. These findings indicate that constitutive Tudor domain-mediated and damage-specific BRCT domain-phospho-H2A-dependent interactions of Rad9 with chromatin cooperate to establish G1 checkpoint arrest.

Monoubiquitination and deubiquitination of FANCD2:FANCI heterodimer is central to DNA repair in a pathway that is defective in the cancer predisposition syndrome Fanconi anemia (FA). The "FA core complex" contains the RING-E3 ligase FANCL and seven other essential proteins that are mutated in various FA subtypes. Here, we purified recombinant FA core complex to reveal the function of these other proteins. The complex contains two spatially separate FANCL molecules that are dimerized by FANCB and FAAP100. FANCC and FANCE act as substrate receptors and restrict monoubiquitination to the FANCD2:FANCI heterodimer in only a DNA-bound form. FANCA and FANCG are dispensable for maximal in vitro ubiquitination. Finally, we show that the reversal of this reaction by the USP1:UAF1 deubiquitinase only occurs when DNA is disengaged. Our work reveals the mechanistic basis for temporal and spatial control of FANCD2:FANCI monoubiquitination that is critical for chemotherapy responses and prevention of Fanconi anemia.

ATM/ATR-like protein kinases play central roles in the maintenance of genome stability and phosphorylate numerous substrates in response to DNA damage, preferentially on SQ or TQ motifs. ATM/ATR substrates often contain several closely spaced SQ/TQ motifs in regions that have been termed SQ/TQ cluster domains (SCDs). SCDs are now considered a structural hallmark of DNA-damage-response proteins. Mutational analyses of a number of SCD-containing proteins indicate that multisite phosphorylation of SQ/TQ motifs is required for normal DNA-damage responses, most commonly by mediating protein-protein interactions in the formation of DNA-damage-induced complexes. SCD sequences are highly diverse and these domains may be largely unfolded in their native state rather than adopting a common three-dimensional fold. Structural disorder of SCDs could be advantageous for efficient phosphorylation by ATM/ATR kinases and also enable them to be molded into distinct conformations to facilitate flexible interactions with multiple binding partners.

S100A1, a Ca2؉ -sensing protein of the EF-hand family, is most highly expressed in myocardial tissue, and cardiac S100A1 overexpression in vitro has been shown to enhance myocyte contractile properties. To study the physiological consequences of S100A1 in vivo, transgenic mice were developed with cardiac-restricted overexpression of S100A1. Characterization of two independent transgenic mouse lines with ϳ4-fold overexpression of S100A1 in the myocardium revealed a marked augmentation of in vivo basal cardiac function that remained elevated after ␤-adrenergic receptor stimulation. Contractile function and Ca 2؉ handling properties were increased in ventricular cardiomyocytes isolated from S100A1 transgenic mice. Enhanced cellular Ca 2؉ cycling by S100A1 was associated both with increased sarcoplasmic reticulum Ca 2؉ content and enhanced sarcoplasmic reticulum Ca 2؉ -induced Ca 2؉ release, and S100A1 was shown to associate with the cardiac ryanodine receptor. No alterations in ␤-adrenergic signal transduction or major cardiac Ca 2؉ -cycling proteins occurred, and there were no signs of hypertrophy with chronic cardiac S100A1 overexpression. Our findings suggest that S100A1 plays an important in vivo role in the regulation of cardiac function perhaps through interacting with the ryanodine receptor. Because S100A1 protein expression is downregulated in heart failure, increasing S100A1 expression in the heart may represent a novel means to augment contractility.

A heterotrimeric member of the AMP-activated protein kinase (AMPK) isoenzyme family was purified from rat skeletal muscle by immunoaffinity chromatography, consisting of an K K2 catalytic and two non-catalytic subunits, L L2 and Q Q1. The AMPK L L2 cDNA (271 amino acids (aa), molecular weight (MW) = 30 307, pI 6.3) was cloned from skeletal muscle and found to share an overall identity of 70% with L L1 (270 aa, MW = 30 475, pI 6.0). In the liver AMPK L L1 subunit, Ser-182 is constitutively phosphorylated whereas in skeletal muscle L L2 isoform, we find that Ser-182 is only partially phosphorylated. In addition, the autophosphorylation sites Ser-24, Ser-25 found in the L L1 are replaced by Ala-Glu in the L L2 isoform. L L2 contains seven more Ser and one less Thr residues than L L1, raising the possibility of differential post-translational regulation. Immunoblot analysis further revealed that soleus muscle (slow twitch) contains exclusively L L1 associated with K K2, whereas extensor digitorum longus muscle K K2 (EDL, fast twitch) associates with L L2 as well as L L1. Sequence analysis revealed that glycogen synthase, a known AMPK substrate, co-immunoprecipitated with the AMPK K K2L L2Q Q1 complex. z 1999 Federation of European Biochemical Societies.

Ca2؉ signaling plays a central role in cardiac contractility and adaptation to increased hemodynamic demand. We have generated mice with a targeted deletion of the S100A1 gene coding for the major cardiac isoform of the large multigenic S100 family of EF hand Ca 2؉ -binding proteins. S100A1 ؊/؊ mice have normal cardiac function under baseline conditions but have significantly reduced contraction rate and relaxation rate responses to ␤-adrenergic stimulation that are associated with a reduced Ca 2؉ sensitivity. In S100A1 ؊/؊ mice, basal left-ventricular contractility deteriorated following 3-week pressure overload by thoracic aorta constriction despite a normal adaptive hypertrophy. Surprisingly, heterozygotes also had an impaired response to acute ␤-adrenergic stimulation but maintained normal contractility in response to chronic pressure overload that coincided with S100A1 upregulation to wild-type levels. In contrast to other genetic models with impaired cardiac contractility, loss of S100A1 did not lead to cardiac hypertrophy or dilation in aged mice. The data demonstrate that high S100A1 protein levels are essential for the cardiac reserve and adaptation to acute and chronic hemodynamic stress in vivo.

Nuclear Rad51 focus formation is required for homologydirected repair of DNA double-strand breaks (DSBs), but its regulation in response to non-DSB lesions is poorly understood. Here we report a novel human SQ/TQ cluster domain-containing protein termed ASCIZ that forms Rad51-containing foci in response to base-modifying DNA methylating agents but not in response to DSB-inducing agents. ASCIZ foci seem to form prior to Rad51 recruitment, and an ASCIZ core domain can concentrate Rad51 in focus-like structures independently of DNA damage. ASCIZ depletion dramatically increases apoptosis after methylating DNA damage and impairs Rad51 focus formation in response to methylating agents but not after ionizing radiation. ASCIZ focus formation and increased apoptosis in ASCIZ-depleted cells depend on the mismatch repair protein MLH1. Interestingly, ASCIZ foci form efficiently during G1 phase, when sister chromatids are unavailable as recombination templates. We propose that ASCIZ acts as a lesion-specific focus scaffold in a Rad51-dependent pathway that resolves cytotoxic repair intermediates, most likely single-stranded DNA gaps, resulting from MLH1-dependent processing of base lesions.