Danna B. Zimmer

The S100 protein family: history, function, and expression.

The S100 family of calcium binding proteins contains approximately 16 members each of which exhibits a unique pattern of tissue/cell type specific expression. Although the distribution of these proteins is not restricted to the nervous system, the implication of several members of this family in nervous system development, function, and disease has sparked new interest in these proteins. We now know that the original two members of this family, S100A1 and S100B, can regulate a diverse group of cellular functions including cell-cell communication, cell growth, cell structure, energy metabolism, contraction and intracellular signal transduction. Although some members of the family may function extracellularly, most appear to function as intracellular calcium-modulated proteins and couple extracellular stimuli to cellular responses via interaction with other cellular proteins called target proteins. Interaction of these proteins with target proteins appear to involve cysteine residues (one in S100A1 and two in S100B), as well as a stretch of 13 amino acids, in the middle of the molecule called the linker region, which connects the two EF-hand calcium binding domains. In addition to the amino acid sequence and secondary structures of these proteins, the structures of the genes encoding these proteins are highly conserved. Studies on the expression of these proteins have demonstrated that a complex mixture of transcriptional and postranscriptional mechanisms regulate S100 expression. Further analysis of the function and expression of these proteins in both nervous and nonnervous tissues will provide important information regarding the role of altered S100 expression in nervous system development, function and disease.

S100 proteins in cancer

In humans, the S100 protein family is composed of 21 members that exhibit a high degree of structural similarity, but are not functionally interchangeable. This family of proteins modulates cellular responses by functioning both as intracellular Ca2+ sensors and as extracellular factors. Dysregulated expression of multiple members of the S100 family is a common feature of human cancers, with each type of cancer showing a unique S100 protein profile or signature. Emerging in vivo evidence indicates that the biology of most S100 proteins is complex and multifactorial, and that these proteins actively contribute to tumorigenic processes such as cell proliferation, metastasis, angiogenesis and immune evasion. Drug discovery efforts have identified leads for inhibiting several S100 family members, and two of the identified inhibitors have progressed to clinical trials in patients with cancer. This Review highlights new findings regarding the role of S100 family members in cancer diagnosis and treatment, the contribution of S100 signalling to tumour biology, and the discovery and development of S100 inhibitors for treating cancer.

S100A1 and Calmodulin Compete for the Same Binding Site on Ryanodine Receptor.

In heart and skeletal muscle an S100 protein family member, S100A1, binds to the ryanodine receptor (RyR) and promotes Ca2+ release. Using competition binding assays, we further characterized this system in skeletal muscle and showed that Ca2+-S100A1 competes with Ca2+-calmodulin (CaM) for the same binding site on RyR1. In addition, the NMR structure was determined for Ca2+-S100A1 bound to a peptide derived from this CaM/S100A1 binding domain, a region conserved in RyR1 and RyR2 and termed RyRP12 (residues 3616-3627 in human RyR1). Examination of the S100A1-RyRP12 complex revealed residues of the helical RyRP12 peptide (Lys-3616, Trp-3620, Lys-3622, Leu-3623, Leu-3624, and Lys-3626) that are involved in favorable hydrophobic and electrostatic interactions with Ca2+-S100A1. These same residues were shown previously to be important for RyR1 binding to Ca2+-CaM. A model for regulating muscle contraction is presented in which Ca2+-S100A1 and Ca2+-CaM compete directly for the same binding site on the ryanodine receptor.

S100A1 Binds to the Calmodulin-binding Site of Ryanodine Receptor and Modulates Skeletal Muscle Excitation-Contraction Coupling

S100A1, a 21-kDa dimeric Ca2+-binding protein, is an enhancer of cardiac Ca2+ release and contractility and a potential therapeutic agent for the treatment of cardiomyopathy. The role of S100A1 in skeletal muscle has been less well defined. Additionally, the precise molecular mechanism underlying S100A1 modulation of sarcoplasmic reticulum Ca2+ release in striated muscle has not been fully elucidated. Here, utilizing a genetic approach to knock out S100A1, we demonstrate a direct physiological role of S100A1 in excitation-contraction coupling in skeletal muscle. We show that the absence of S100A1 leads to decreased global myoplasmic Ca2+ transients following electrical excitation. Using high speed confocal microscopy, we demonstrate with high temporal resolution depressed activation of sarcoplasmic reticulum Ca2+ release in S100A1-/- muscle fibers. Through competition assays with sarcoplasmic reticulum vesicles and through tryptophan fluorescence experiments, we also identify a novel S100A1-binding site on the cytoplasmic face of the intact ryanodine receptor that is conserved throughout striated muscle and corresponds to a previously identified calmodulin-binding site. Using a 12-mer peptide of this putative binding domain, we demonstrate low micromolar binding affinity to S100A1. NMR spectroscopy reveals this peptide binds within the Ca2+-dependent hydrophobic pocket of S100A1. Taken together, these data suggest that S100A1 plays a significant role in skeletal muscle excitation-contraction coupling, primarily through specific interactions with a conserved binding domain of the ryanodine receptor. This warrants further investigation into the use of S100A1 as a therapeutic target for the treatment of both cardiac and skeletal myopathies.

Identification of an S100A1/S100B target protein: phosphoglucomutase

Phosphoglucomutase was identified as a potential intracellular S100 target protein because it interacted with two members of the S100 family of calcium-modulated proteins, S100A1 and S100B, in gel overlay experiments. These results were confirmed by affinity chromatography experiments demonstrating that S100A1 and S100B bound to phosphoglucomutase-Sepharose in a calcium-dependent manner. In the reverse experiment, phosphoglucomutase bound to S100A1 and S100B-Sepharose in a calcium-dependent manner. S100A1 inhibited phosphoglucomutase activity in a calcium-dependent manner. In contrast, S100B stimulated phosphoglucomutase activity in a calcium-dependent manner. Other calcium-binding proteins (calmodulin, troponin C, parvalbumin, and alpha-lactalbumin) had no effect on phosphoglucomutase. These results suggest that the effects of S100A1 and S100B are not nonspecific effects of low molecular weight, acidic proteins. This is the first report of an S100 target protein whose activity is antagonistically regulated by S100A1 and S100B, suggesting that cellular diversity in intracellular calcium signaling pathways may be due, at least in part, to the complement of S100 proteins expressed in different cell types.

S100A1: Structure, Function, and Therapeutic Potential

S100A1 is a member of the S100 family of calcium-binding proteins. As with most S100 proteins, S100A1 undergoes a large conformational change upon binding calcium as necessary to interact with numerous protein targets. Targets of S100A1 include proteins involved in calcium signaling (ryanidine receptors 1 & 2, Serca2a, phopholamban), neurotransmitter release (synapsins I & II), cytoskeletal and filament associated proteins (CapZ, microtubules, intermediate filaments, tau, mocrofilaments, desmin, tubulin, F-actin, titin, and the glial fibrillary acidic protein GFAP), transcription factors and their regulators (e.g. myoD, p53), enzymes (e.g. aldolase, phosphoglucomutase, malate dehydrogenase, glycogen phosphorylase, photoreceptor guanyl cyclases, adenylate cyclases, glyceraldehydes-3-phosphate dehydrogenase, twitchin kinase, Ndr kinase, and F1 ATP synthase), and other Ca2+-activated proteins (annexins V & VI, S100B, S100A4, S100P, and other S100 proteins). There is also a growing interest in developing inhibitors of S100A1 since they may be beneficial for treating a variety of human diseases including neurological diseases, diabetes mellitus, heart failure, and several types of cancer. The absence of significant phenotypes in S100A1 knockout mice provides some early indication that an S100A1 antagonist could have minimal side effects in normal tissues. However, development of S100A1-mediated therapies is complicated by S100A1s unusual ability to function as both an intracellular signaling molecule and as a secreted protein. Additionally, many S100A1 protein targets have only recently been identified, and so fully characterizing both these S100A1-target complexes and their resulting functions is a necessary prerequisite.

Isolation of a rat S100α cDNA and distribution of its mRNA in rat tissues

Abstract In order to clarify the reported discrepancies in S100α protein and mRNA distribution in rat tissues, a rat S100α cDNA has been isolated and this species homologous probe along with a rat S100β cDNA probe has been used to examine S100 mRNA expression in rat tissues. Although the rat S100α cDNA was missing approximately 30 nucleotides of coding sequence, only 4 conservative changes in amino acid sequence were observed when the deduced amino acid sequence was compared to the bovine S100α amino acid sequence. Thus, S100α proteins, like S100β proteins, are highly conserved among species. All nineteen of the tissues examined (including cerebrum and cerebellum) contained S100α mRNA. In addition, S100β mRNA was detected in thirteen of the nineteen tissues examined. These results are in agreement with previous protein distribution studies and further demonstrate that S100 proteins are not brain-specific and are expressed in a large number of tissues. Although S100α and S100β mRNAs were detected in rat tissues which had previously been reported to contain S100α and S100β protein, a direct correlation between the protein and mRNA levels were not observed, suggesting that different mechanisms regulate S100 expression in various tissues. S100α exhibited a single similar size mRNA species (0.5 Kb) in all tissues examined, as did S100β (1.5 Kb), suggesting that the individual S100 proteins are expressed as single mRNA and protein products in rat tissues.

The Calcium-Dependent Interaction of S100B with Its Protein Targets

S100B is a calcium signaling protein that is a member of the S100 protein family. An important feature of S100B and most other S100 proteins (S100s) is that they often bind Ca2+ ions relatively weakly in the absence of a protein target; upon binding their target proteins, Ca2+-binding then increases by as much as from 200- to 400-fold. This manuscript reviews the structural basis and physiological significance of increased Ca2+-binding affinity in the presence of protein targets. New information regarding redundancy among family members and the structural domains that mediate the interaction of S100B, and other S100s, with their targets is also presented. It is the diversity among individual S100s, the protein targets that they interact with, and the Ca2+ dependency of these protein-protein interactions that allow S100s to transduce changes in [Ca2+]intracellular levels into spatially and temporally unique biological responses.

The myocardial protein S100A1 plays a role in the maintenance of normal gene expression in the adult heart.

S100A1 and S100B are members of a family of 20 kDa Ca2++-binding homodimers that play a role in signal transduction in mammalian cells. S100A1 is the major isoform in normal heart and S100B, normally a brain protein, is induced in hypertrophic myocardium and functions as an intrinsic negative modulator of the hypertrophic response. In order to examine the function of S100A1, we first showed that, in contrast to S100B, S100A1 was downregulated in rat experimental models of myocardial hypertrophy following myocardial infarction or pressure overload. Second, in co-transfection experiments in cultured neonatal rat cardiac myocytes, S100A1 inhibited the α1-adrenergic activation of promoters of genes induced during the hypertrophic response including the fetal genes skeletal α actin (skACT), and β-myosin heavy chain (MHC) and S100B, but not the triiodothyronine (T3) activation of the promoter of the α-MHC gene, that is normally expressed in adult myocardium. These results suggest that S100A1 is involved in the maintenance of the genetic program that defines normal myocardial function and that its downregulation is permissive for the induction of genes that underlie myocardial hypertrophy.

Structure of human brain fructose 1,6-(bis)phosphate aldolase: linking isozyme structure with function

Fructose‐1,6‐(bis)phosphate aldolase is a ubiquitous enzyme that catalyzes the reversible aldol cleavage of fructose‐1,6‐(bis)phosphate and fructose 1‐phosphate to dihydroxyacetone phosphate and either glyceral‐dehyde‐3‐phosphate or glyceraldehyde, respectively. Vertebrate aldolases exist as three isozymes with different tissue distributions and kinetics: aldolase A (muscle and red blood cell), aldolase B (liver, kidney, and small intestine), and aldolase C (brain and neuronal tissue). The structures of human aldolases A and B are known and herein we report the first structure of the human aldolase C, solved by X‐ray crystallography at 3.0 Å resolution. Structural differences between the isozymes were expected to account for isozyme‐specific activity. However, the structures of isozymes A, B, and C are the same in their overall fold and active site structure. The subtle changes observed in active site residues Arg42, Lys146, and Arg303 are insufficient to completely account for the tissue‐specific isozymic differences. Consequently, the structural analysis has been extended to the isozyme‐specific residues (ISRs), those residues conserved among paralogs. A complete analysis of the ISRs in the context of this structure demonstrates that in several cases an amino acid residue that is conserved among aldolase C orthologs prevents an interaction that occurs in paralogs. In addition, the structure confirms the clustering of ISRs into discrete patches on the surface and reveals the existence in aldolase C of a patch of electronegative residues localized near the C terminus. Together, these structural changes highlight the differences required for the tissue and kinetic specificity among aldolase isozymes.