S100 Ca2+-binding proteins became of major interest because of their differential expression in tissues and their association with human diseases. Earlier studies showed that 13 S100 genes are located as a cluster on human chromosome 1q21. Since a number of mouse S 100 genes, such as S100A4 and S100A6, have been localized to a syntenic region on mouse chromosome 3, we investigated if the S100 gene cluster exists in mouse and is structurally conserved during evolution. First we identified the cDNA sequences of mouse S100A1, S100A3 and S100A5. Then we isolated a 490 kb mouse YAC clone which gives a specific signal by FISH most likely on chromosome 3. Hybridization studies with different mouse S100 cDNAs revealed that eight mouse S100 genes are arranged in a clustered organization similar to that in human. The linkage relationships between the genes S100A8-S100A9 and S100A3-S100A4-S100A5-S100A6 were conserved during divergence of human and mouse about 70 million years ago. However, the separation of the mouse S100 genes S100A1 and S100A13 in comparison to the human linkage group suggests rearrangement processes between human and mouse. Our data demonstrate that the S100 gene cluster is structurally conserved during evolution. Further studies on the genomic organization of the S100 genes including various species could generate new insights into gene regulatory processes and phylogenetic relationships.

Formation of eukaryotic ribosomes requires more than 150 biogenesis factors which transiently interact with the nascent ribosomal subunits. Previously, many pre-ribosomal intermediates could be distinguished by their protein composition and rRNA precursor (pre-rRNA) content. We purified complexes of ribosome biogenesis factors from yeast cells in which de novo synthesis of rRNA precursors was down-regulated by genetic means. We compared the protein composition of these largely pre-rRNA free assemblies with the one of analogous pre-ribosomal preparations by semi-quantitative mass spectrometry. The experimental setup minimizes the possibility that the analysed pre-rRNA free protein modules were derived from (partially) disrupted pre-ribosomal particles and provides thereby strong evidence for their pre-ribosome independent existence. In support of the validity of this approach (i) the predicted composition of the analysed protein modules was in agreement with previously described rRNA-free complexes and (ii) in most of the cases we could identify new candidate members of reported protein modules. An unexpected outcome of these analyses was that free large ribosomal subunits are associated with a specific set of ribosome biogenesis factors in cells where neo-production of nascent ribosomes was blocked. The data presented strengthen the idea that assembly of eukaryotic pre-ribosomal particles can result from transient association of distinct building blocks.

S100 proteins became of major interest because of their divergent cell-and tissue-specific expression, their close association with a number of human diseases, and their importance for clinical diagnostics. Here, we report for the first time the purification and characterization of human recombinant S100A13. Flow dialysis revealed that the homodimeric S100A13 binds four Ca 2؉ in two sets of binding sites, both displaying positive cooperativity but of very different affinity. Fluorescence and difference spectrophotometry indicate that the Trp/Tyr signal changes are almost complete upon binding of Ca 2؉ to the two high affinity sites, which probably correspond to the C-terminal EF-hands in each subunit. The far-UV circular dichroic signal also changes upon binding of the first two Ca 2؉ . So far, the tissue distribution of S100A13 has not been well characterized. Here, we show that S100A13 is widely expressed in various types of tissues with a high expression level in thyroid gland. Using specific antisera against S100A13, high protein expression was detected in follicle cells of thyroid, Leydig cells of testis, and specific cells of brain. In human smooth muscle cells, which co-express S100A2 in the nucleus and S100A1 in stress fibers, S100A13 shows a unique subcellular localization in the perinuclear area. These data suggest diverse functions for this protein in signal transduction.The S100 protein family is characterized by two sets of different EF-hands in the dimer and currently numbers at least 18 different members. In contrast to the ubiquitously expressed calmodulin, the expression of S100 proteins is cell-and tissuespecific. Some are expressed mainly in a few specific tissues, such as S100A3 in human hair cuticle cells (1, 2) or S100A8/ A9/A12 in myeloid cells (e.g. monocytes and granulocytes) (3-5). Other members are detected in a broader range of tissues, such as S100A2 in lung, kidney, liver, cardiac, and skeletal muscle (6 -8), or S100A6, which is overexpressed in many tumor cells (9, 10). S100 proteins show different binding affinities for Ca 2ϩ , Cu 2ϩ , and Zn 2ϩ , yielding new clues to their functional significance in cellular events (11). In general, they can bind four Ca 2ϩ per dimer and display rather low affinities, with [Ca 2ϩ ] 0.5 values of 100 -300 M. Some parameters such as high ionic strength can decrease the Ca 2ϩ affinity, e.g. in S100A1 (12) and S100A6 (13,14). In contrast, Zn 2ϩ increases the Ca 2ϩ affinity of S100A12 (15) and S100B (12) and enhances the calcium-dependent association of S100B to target peptides (16). Recently, the Ndr kinase was shown to be a target protein of S100B whereby the interaction is regulated by changes in the intracellular calcium concentration (17). Hence, calcium seems to be an important regulator of S100 interactions with target proteins.The cDNA of human and murine S100A13 was first identified by screening expressed sequence tag data bases (18). 15 genes coding for S100 proteins, including two epidermal differentiation genes with a S100 domain, are a...