Proteins share peptidic sequences, such as a nuclear localization signal (NLS), which guide them to particular membranebound compartments. Similarities have also been observed within different classes of signals that target proteins to membrane-less subnuclear compartments. Common localization signals affect spatial and temporal subcellular organization and are thought to allow the coordinated response of different molecular networks to a given signaling cue. Here we identify a higher-order and predictive code, {[RR(I/L)X 3 r] (n, n>1) ؉[L(/N)(V/L)] (n,n>1) }, that establishes high-affinity interactions between a group of proteins and the nucleolus in response to a specific signal. This position-independent code is referred to as a nucleolar detention signal regulated by H ؉ (NoDS H؉ ) and the class of proteins includes the cIAP2 apoptotic regulator, VHL ubiquitylation factor, HSC70 heat shock protein and RNF8 transcription regulator. By identifying a common subnuclear targeting consensus sequence, our work reveals rules governing the dynamics of subnuclear organization and ascribes new modes of regulation to several proteins with diverse steady-state distributions and dynamic properties. INTRODUCTIONBiochemical processes occurring in the nucleus contribute to its compartmentalization, which facilitates the control of molecular networks (Chubb and Bickmore, 2003). Unlike the cytoplasm, compartmentalization in the nucleus does not rely on the concentration of molecules behind membranes to enhance biochemical reactions. Similar gene loci and regulatory proteins are concentrated within specific membraneless nuclear substructures such as speckles, PML bodies, Cajal bodies, and nucleoli (Misteli, 2001(Misteli, , 2004Chubb and Bickmore, 2003;Isogai and Tjian, 2003;Zimber et al., 2004).The temporal and spatial precision of intranuclear dynamics of nucleic acids and polypeptides is central to the proper control of the cell cycle, transcription, apoptosis, ubiquitylation, ribosomal biogenesis, and several other pathways (Shou et al., 1999;Visintin et al., 1999;Weber et al., 1999;Dundr et al., 2000Dundr et al., , 2004Barseguian et al., 2002;Wong et al., 2002;Isogai and Tjian, 2003;Leung et al., 2004;Zaidi et al., 2005). For example, silent gene loci at the nuclear periphery can move centripetally when activated to be repositioned away from repressive factors and closer to activators concentrated within particular subnuclear compartments (Kosak and Groudine, 2004;Misteli, 2004).Several molecules rely on common peptidic sequences to localize to a given membrane-bound compartment. This includes nuclear localization/export signals (NLS or NES, respectively;Conti and Izaurralde, 2001;Kutay and Guttinger, 2005; and cell membrane localization signals (Shikano et al., 2005). Identification of such sequences has been instrumental in the functional characterization of a very large number of proteins. Some similarities have also been observed within each class of subnuclear targeting signal. This has been the case for the nucleolus...

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SUMMARY Numerous fish species, including rainbow trout (Oncorhynchus mykiss), are able to inhabit both freshwater and seawater and routinely migrate between the two environments. One of the most critical adjustments allowing such successful migrations is a remodelling of the gill in which a suite of morphological and molecular changes ensure optimal function in the face of reversing requirements for salt and water balance. The remodelling leads to specific freshwater and seawater gill phenotypes that are readily identified by the orientation and/or quantities of specific ion transporters and the presence or absence of specific cell types. The proximate cues promoting gill phenotypic plasticity are unknown. Here, by assessing the consequences of a salt-enriched diet (in the absence of any changes in external salinity) in the freshwater rainbow trout, we demonstrate that internal salt loading alone, is able to induce various elements of the seawater gill phenotype. Specifically, we show upregulation of three ion transport genes, cystic fibrosis transmembrane conductance regulator (CFTR),Na+/K+/2Cl- co-transporter (NKCC1) and Na+/K+-ATPase, which are essential for ionic regulation in seawater, and the appearance of chloride cell-accessory cell complexes,which are normally restricted to fish inhabiting seawater. These data provide compelling evidence that gill remodelling during migration from freshwater to seawater may involve sensing of elevated levels of internal salt.

The goal of this study was to determine the effect of the changes in gill morphology induced by dietary salt feeding on several aspects of gill function in rainbow trout Oncorhynchus mykiss maintained in fresh water with specific emphasis on Cl(-) uptake (J(IN)Cl(-)) and acid-base regulation. The addition of 11% NaCl to the diet caused J(IN)Cl(-) to be reduced by c. 45% from 214·4 ± 26·7 to 117·3 ± 17·4 µmol kg(-1) h(-1) (mean ± s.e.). Rates of Cl(-) efflux (J(OUT)Cl(-)), net Cl(-) flux (J(NET)Cl(-)), J(NET) Na(+) and plasma levels of Na(+) or Cl(-) were unaffected by salt feeding. On the basis of significant effect of the salt diet on decreasing the maximal uptake rate of Cl(-)(J(MAX)Cl(-)), it would appear that internal salt loading caused a decrease in the number of functional ion transport proteins involved in Cl(-) uptake (e.g. Cl(-) -HCO(3)(-) exchangers) and decreased the transporting capacity of existing proteins. The acid-base regulating capacity of control fish and salt-loaded fish was assessed by monitoring arterial blood acid-base status [partial pressure of CO(2) (PCO(2)), pH and HCO(3)(-)] during exposure to external hypercapnia (nominally 7·5 mm Hg). Both groups of fish exhibited typical compensatory responses to sustained hypercapnia consisting of the gradual accumulation of plasma HCO(3) (-) and thus metabolic restoration within 24 h of the initial respiratory acidosis elicited by hypercapnia. Overall, the results demonstrate that while Cl(-) uptake capacity is reduced in salt-fed fish, there is no associated alteration in acid-base regulating capability.