Rhodanese domains are ubiquitous structural modules occurring in the three major evolutionary phyla. They are found as tandem repeats, with the C-terminal domain hosting the properly structured active-site Cys residue, as single domain proteins or in combination with distinct protein domains. An increasing number of reports indicate that rhodanese modules are versatile sulfur carriers that have adapted their function to fulfill the need for reactive sulfane sulfur in distinct metabolic and regulatory pathways. Recent investigations have shown that rhodanese domains are also structurally related to the catalytic subunit of Cdc25 phosphatase enzymes and that the two enzyme families are likely to share a common evolutionary origin. In this review, the rhodanese/Cdc25 phosphatase superfamily is analyzed. Although the identification of their biological substrates has thus far proven elusive, the emerging picture points to a role for the amino-acid composition of the active-site loop in substrate recognition/specificity. Furthermore, the frequently observed association of catalytically inactive rhodanese modules with other protein domains suggests a distinct regulatory role for these inactive domains, possibly in connection with signaling.

The conserved topological structure observed in various molecular families such as globins or cytochromes c allows structural equivalencing of residues in every homologous structure and defines in a coherent way a global alignment in each sequence family. A search was performed for equivalent residue pairs in various topological families that were buried in protein cores or exposed at the protein surface and that had mutated but maintained similar unmutated environments. Amino acid residues with atoms in contact with the mutated residue pairs defined the environment. Matrices of preferred amino acid exchanges were then constructed and preferred or avoided amino acid substitutions deduced. Given the conserved atomic neighborhoods, such natural in vivo substitutions are subject to similar constraints as point mutations performed in site-directed mutagenesis experiments. The exchange matrices should provide guidelines for "safe" amino acid substitutions least likely to disturb the protein structure, either locally or in its overall folding pathway, and most likely to allow probing of the structural and functional significance of the substituted site.

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The number of water molecules which are expected to be experimentally located by protein crystallography was determined by multiple regression analysis on a test set of 873 known protein crystal structures determined at room temperature and on another set of 33 structures determined at low temperature. The dependence of the number of water molecules included in the protein models as a function of a number of significant regressors, such as resolution, fraction of crystal volume occupied by the solvent, number of residues in the asymmetric unit, fraction of apolar protein surface or secondary structure, has been studied. The number of water molecules included in crystallographic models depends primarily on the resolution at which the structure has been solved, while the temperature of the data collection has only marginal influence. On average, at 2.0 A resolution one water molecule per residue is included in the model, while at 1.0 A resolution about 1.6-1.7 are crystallographically located. At 2.0 A resolution the well known rule-of-thumb of 'one water per protein residue' is confirmed, though the number of water molecules experimentally observed is strongly dependent on resolution. The results presented are useful in assessing the quality of a protein crystal structure, in selecting structural results to be compared and in evaluating the expected improvement on the solvent structure when increasing the crystallographic resolution.

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NK cells discriminate between normal and abnormal cells by means of surface inhibitory receptors able to Universita `di Genova Via L.B. Alberti 1 detect human leukocyte antigen (HLA) class I molecules. Cells expressing normal amounts of HLA class I mole-16132 Genova Italy cules are protected from NK cell-mediated cell lysis, whereas cells that display lowered levels of HLA class 2 Istituto Giannina Gaslini Largo G. Gaslini 5 I expression are recognized and efficiently killed by NK cells (Biron, 1997; Moretta et al., 1996, 2001). In fact, 16147 Genova Italy downregulation of HLA class I expression is often induced by tumor transformation or viral infection (Gar-