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SummaryHuman serum albumin (HSA), the most prominent protein in plasma, binds different classes of ligands at multiple sites. HSA provides a depot for many compounds, affects pharmacokinetics of many drugs, holds some ligands in a strained orientation providing their metabolic modification, renders potential toxins harmless transporting them to disposal sites, accounts for most of the antioxidant capacity of human serum, and acts as a NO-carrier. The globular domain structural organization of monomeric HSA is at the root of its allosteric properties which are reminiscent of those of multimeric proteins. Here, structural, functional, biotechnological, and biomedical aspects of ligand binding to HSA are summarized. IUBMB Life, 57: 787 -796, 2005

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Neuroglobin, mainly expressed in vertebrate brain and retina, is a recently identified member of the globin superfamily. Augmenting O(2) supply, neuroglobin promotes survival of neurons upon hypoxic injury, potentially limiting brain damage. In the absence of exogenous ligands, neuroglobin displays a hexacoordinated heme. O(2) and CO bind to the heme iron, displacing the endogenous HisE7 heme distal ligand. Hexacoordinated human neuroglobin displays a classical globin fold adapted to host the reversible bis-histidyl heme complex and an elongated protein matrix cavity, held to facilitate O(2) diffusion to the heme. The neuroglobin structure suggests that the classical globin fold is endowed with striking adaptability, indicating that hemoglobin and myoglobin are just two examples within a wide and functionally diversified protein homology superfamily.

Steroid hormones exert profound effects on cell growth, development, differentiation, and homeostasis. Their effects are mediated through specific intracellular steroid receptors that act via multiple mechanisms. Among others, the action mechanism starting upon 17 -estradiol (E2) binds to its receptors (ER) is considered a paradigmatic example of how steroid hormones function. Ligand-activated ER dimerizes and translocates in the nucleus where it recognizes specific hormone response elements located in or near promoter DNA regions of target genes. Behind the classical genomic mechanism shared with other steroid hormones, E2 also modulates gene expression by a second indirect mechanism that involves the interaction of ER with other transcription factors which, in turn, bind their cognate DNA elements. In this case, ER modulates the activities of transcription factors such as the activator protein (AP)-1, nuclear factor-B (NF-B) and stimulating protein-1 (Sp-1), by stabilizing DNA-protein complexes and/or recruiting co-activators. In addition, E2 binding to ER may also exert rapid actions that start with the activation of a variety of signal transduction pathways (e.g. ERK/MAPK, p38/MAPK, PI3K/AKT, PLC/PKC). The debate about the contribution of different ER-mediated signaling pathways to coordinate the expression of specific sets of genes is still open. This review will focus on the recent knowledge about the mechanism by which ERs regulate the expression of target genes and the emerging field of integration of membrane and nuclear receptor signaling, giving examples of the ways by which the genomic and non-genomic actions of ERs on target genes converge.

A fraction of the nuclear estrogen receptor ␣ (ER␣) is localized to the plasma membrane region of 17␤-estradiol (E2) target cells. We previously reported that ER␣ is a palmitoylated protein. To gain insight into the molecular mechanism of ER␣ residence at the plasma membrane, we tested both the role of palmitoylation and the impact of E2 stimulation on ER␣ membrane localization. The cancer cell lines expressing transfected or endogenous human ER␣ (HeLa and HepG2, respectively) or the ER␣ nonpalmitoylable Cys447Ala mutant transfected in HeLa cells were used as experimental models. We found that palmitoylation of ER␣ enacts ER␣ association with the plasma membrane, interaction with the membrane protein caveolin-1, and nongenomic activities, including activation of signaling pathways and cell proliferation (i.e., ERK and AKT activation, cyclin D 1 promoter activity, DNA synthesis). Moreover, E2 reduces both ER␣ palmitoylation and its interaction with caveolin-1, in a time-and dose-dependent manner. These data point to the physiological role of ER␣ palmitoylation in the receptor localization to the cell membrane and in the regulation of the E2-induced cell proliferation. INTRODUCTIONThe sex steroid 17␤-estradiol (E2) acts by binding to its nuclear receptors (i.e., ER␣ and ER␤) that then transactivate target genes. In addition, E2 induces rapid, nongenomic actions involving plasma membrane-associated signaling that require a membrane ER (Coleman and Smith, 2001;Kelly and Levin, 2001;Jakacka et al., 2002;Marino et al., 2002). Although different structural and functional properties have been reported for the membrane-associated ER by comparison with nuclear ER␣ and ER␤ (Ropero et al., 2002;Toran-Allerand et al., 2002;Deecher et al., 2003), immunocytochemical studies revealed the presence of a significant fraction of nuclear ER also on the plasma membrane (Pappas et al., 1995;Norfleet et al., 1999;Dan et al., 2003;Razandi et al., 2003;Arvanitis et al., 2004;Song et al., 2004). In addition, a single mRNA originates a similarly sized nuclear and membrane ER in ER␣-transfected Chinese hamster ovary and HeLa cells (Razandi et al., 1999;Marino et al., 2002Marino et al., , 2003. Thus, ER␣ localizes to both the nucleus and the plasma membrane. Moreover, the membrane ER␣ is emerging as the primary endogenous mediator of E2 rapid responses important in cell proliferation (Marino et al., 1998(Marino et al., , 2002Castoria et al., 1999Castoria et al., , 2001Razandi et al., 1999Razandi et al., , 2000Lobenhofer et al., 2000;Acconcia et al., 2004a;Fernando and Wimalasena, 2004).Debate is open regarding the structural bases and the mechanisms for ER␣ maintenance at and translocation to the plasma membrane. ER␣ does not display any intrinsic transmembrane domain (Song et al., 2004); thus, ER␣ interaction with specific membrane proteins have been proposed to explain its membrane localization Migliaccio et al., 2002;Razandi et al., 2002Razandi et al., , 2003Toran-Allerand et al., 2002;Arvanitis et al., 2004). In particular, the Ser522 re...

Macrophage‐generated oxygen‐ and nitrogen‐reactive species control the development of Mycobacterium tuberculosis infection in the host. Mycobacterium tuberculosis ‘truncated hemoglobin’ N (trHbN) has been related to nitric oxide (NO) detoxification, in response to macrophage nitrosative stress, during the bacterium latent infection stage. The three‐dimensional structure of oxygenated trHbN, solved at 1.9 Å resolution, displays the two‐over‐two α‐helical sandwich fold recently characterized in two homologous truncated hemoglobins, featuring an extra N‐terminal α‐helix and homodimeric assembly. In the absence of a polar distal E7 residue, the O2 heme ligand is stabilized by two hydrogen bonds to TyrB10(33). Strikingly, ligand diffusion to the heme in trHbN may occur via an apolar tunnel/cavity system extending for ∼28 Å through the protein matrix, connecting the heme distal cavity to two distinct protein surface sites. This unique structural feature appears to be conserved in several homologous truncated hemoglobins. It is proposed that in trHbN, heme Fe/O2 stereochemistry and the protein matrix tunnel may promote O2/NO chemistry in vivo, as a M.tuberculosis defense mechanism against macrophage nitrosative stress.

PAO specifically oxidizes substrates that have both primary and secondary amino groups. The complex with MDL72527 shows that the primary amino groups are essential for the proper alignment of the substrate with respect to the flavin. Conservation of an N-terminal sequence motif indicates that PAO is member of a novel family of flavoenzymes. Among these, monoamine oxidase displays significant sequence homology with PAO, suggesting a similar overall folding topology.