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DAXX, a modulator of apoptosis and a repressor of basal transcription, was identified in a two-hybrid screen as a protein capable of interacting with a trimeric form of human heat shock factor 1 (HSF1). In human cells, DAXX interacted with HSF1 essentially only during stress, i.e., when factor trimerization occurred. Several lines of experimentation suggested that DAXX is an important mediator of HSF1 activation: (i) overexpression of DAXX enhanced basal transactivation competence of HSF1 in the absence of a stress; (ii) a DAXX fragment exerted dominant-negative effects on HSF1 activation by different types of stress; (ii,) induction of heat shock or stress protein (HSP)70 by heat stress was defective in a cell line lacking functional DAXX; and (iv) RNA interference depletion of DAXX also substantially reduced heat induction of HSF1 activity and HSP70 expression. HSF1 transactivation competence is repressed by an HSP90-containing multichaperone complex that interacts with trimeric factor. Overexpressed HSF1. known to be largely trimeric, only marginally increased HSF1 activity on its own but potentiated the activating effect of DAXX overexpression. Expression of a nonnative protein capable of competing for multichaperone complex also synergistically enhanced activation of HSF1 by DAXX. These observations suggest a model in which DAXX released from its nuclear stores during stress opposes repression of HSF1 transactivation competence by multichaperone complex through its interaction with trimerized HSF1. Our identification of DAXX as a mediator of HSF1 activation raises the question whether DAXX produces some of its pleiotropic effects through modulation of HSP levels. [ABSTRACT FROM AUTHOR]

Here, we describe a new mechanism of host defense that involves the nuclear factors associated with dsRNA (NFAR1 [90 kDa] and NFAR2 [110 kDa]), which constitute part of the shuttling ribonuclear protein (RNP) complex. Activation of the dsRNA-activated protein kinase PKR by viral RNA enabled phosphorylation of NFAR1 and NFAR2 on Thr 188 and Thr 315, an event found to be evolutionarily conserved in Xenopus. Phosphorylated NFAR1 and NFAR2 became dissociated from nuclear factor 45 (NF45), which was requisite for NFAR reshuttling, causing the NFARs to be retained on ribosomes, associate with viral transcripts, and impede viral replication. Cre-loxP animals with depletion of the NFARs in the thymus were exquisitely sensitive to the cytoplasmic replicating virus VSV (vesicular stomatitis virus). Thus, the NFARs constitute a novel, conserved mechanism of host defense used by the cell to detect and impede aberrant translation events. [ABSTRACT FROM AUTHOR]