Cell, Vol. 80, 199-211, January 27, 1995, Copyright © 1995 by Cell Press Transcriptional Regulation Review by Extracellular Signals: Mechanisms and Specificity Caroline S. Hill and Richard Treisman Nuclear Translocation Transcription Laboratory In principle, regulated nuclear localization of transcription Imperial Cancer Research Fund factors can involve regulated activity of either nuclear lo- Lincoln's Inn Fields calization signals (NLSs) or cytoplasmic retention signals, London WC2A 3PX although no well-characterized case of the latter has yet England been reported. N LS activity, which is generally dependent on short regions of basic amino acids, can be regulated either by masking mechanisms or by phosphorylations Changes in cell behavior induced by extracellular signal- within the NLS itself (Hunter and Karin, 1992). For exam- ing molecules such as growth factors and cytokines re- ple, association with an inhibitory subunit masks the NLS quire execution of a complex program of transcriptional of NF-KB and its relatives (Figure 1; for review see Beg events. While the route followed by the intracellular signal and Baldwin, 1993), while an intramolecular mechanism from the cell membrane to its transcription factor targets may mask NLS activity in the heat shock regulatory factor can be traced in an increasing number of cases, how the HSF2 (Sheldon and Kingston, 1993). When transcription specificity of the transcriptional response of the cell to factor localization is dependent on regulated NLS activity, different stimuli is determined is much less clear. How- linkage to a constitutively acting NLS may be sufficient to ever, it is possible to understand at least in principle how render nuclear localization independent of signaling (Beg different stimuli can activate the same signal pathway yet et al., 1992). activate different genes and how small differences in sig- nal strength can generate qualitative differences in gene DNA Binding expression. Many DNA-binding proteins bind DNA as oligomers, and In this review we shall concentrate on transcriptional signals can therefore regulate DNA binding by affecting responses to cell surface receptor-activated signaling factor oligomerization as well as protein-DNA interaction pathways; however, much of our discussion is also appli- itself. For example, signals may induce dissociation of tran- cable to signals induced by environmental stresses or to scription factors from inhibitory molecules that may either extracellular signals that act directly on transcription fac- directly block DNA-binding domain-DNA interactions, as tors, such as steroid hormones. Rather than describe in in the case of certain NF-~<B-h<B complexes (see Beg detail each of the many separate pathways from receptor and Baldwin, 1993), or prevent dimerization of the factor to transcription factor, we have attempted to compare and subunits, as in certain nuclear receptors (see Grone- contrast regulation of a representative set of transcription meyer, 1993). Alternatively, subunit association itself can factors. These factors are introduced in Figure 1, and their be regulated: for example, signal-induced tyrosine phos- properties are summarized in Table 1. To set the stage, phorylation induces STAT (signal transducer and activator we briefly discuss the different properties of transcription of transcription) factor dimerization, probably via mutual factors that can be modified to regulate their behavior phosphotyrosine-SH2 domain interactions (Figure 1; in response to signals. We then use relatively well-charac- Shuai et al., 1994), while heat shock activates HSF (heat terized signaling pathways to illustrate different strategies shock factor) trimerization via an unknown but apparently by which an extracellular stimu lus is converted to an active phosphorylation-independent mechanism (Lis and Wu, transcription factor in the nucleus. Finally, we use these 1993; Westwood and Wu, 1993). examples to illustrate two aspects of specificity in the tran- Since many DNA-binding domains are basic in charac- scriptional response to signals: first, the means by which a ter, phosphorylation at sites within or near DNA-binding given stimulus specifically targets particular transcription domains may prevent DNA binding, perhaps by direct factors and DNA targets; and second, the means by which electrostatic effects. For example, signal-induced dephos- differential responses to qualitatively and quantitatively phorylation of three sites adjacent to the Jun DNA-binding different signals can be generated. domain potentiates DNA binding (Figure 1 ; Binetruy et al., 1991 ; Boyle et al., 1991). DNA-binding properties can also Mechanisms of Regulation be regulated by phosphorylation at remote sites, as in the growth factor-regulated changes in the DNA-binding To activate or repress transcription, transcription factors properties of serum response factor (SRF) (Rivera et al., must be located in the nucleus, bind DNA, and interact 1993) and its accessory factor, the ternary complex factor with the basal transcription apparatus. Accordingly, extra- (TCF) Elk-1 (Figure 1; Gille et al., 1992); however, the cellular signals that regulate transcription factor activity mechanisms underlying such effects remain obscure. may affect one or more of these processes. Most com- monly, regulation is achieved by reversible phosphoryla- Transcriptional Activation tion (for review see Hunter and Karin, 1992). Phosphoryla- A number of transcription factors contain signal-regulated tion of a transcription factor by several different kinases transcriptional activation domains: in these cases, it is pre- (or by a kinase linked to more than one pathway) is a simple sumed that regulated phosphorylation facilitates their in- mechanism that allows different signals to converge at the teraction with the basal transcriptional machinery or co- same factor. activator proteins, although the mechanisms by which this Cell 200 SRF conserved Figure 1. Schematic Diagrams of Transcrip- Ets interaction SP/TP motifs tion Factors That Are Targets For Signaling TCF (Elk-l) Pathways 5133 Transcription factors are depicted by closed CREB bars with relevant domains indicated by stip- Q1 KID Q2 bZIP pling. Where the transcriptionfactor is a mem- S63/$73 T231JS243/S249 ber of a family, a single examplehas been il- lustrated. Hatching, DNA-binding domains; JUN --I ;, :::N\\'~,, 3~ 5 bz~ vertical lines, regulatedphosphorylation sites. SI-I3 S~ Y701 In Elk-l, the three stippled boxes indicatere- STAT 1 gions of homologybetween TCF family mem- bers. In CREB, the glutamine-richdomains that REL HOMOLOGY NLS are absentin CREMisoforms are indicated(Q1 NF-gB (p65) =~\\\\\\\\\\\\\\\\\'~,1 550 and Q2). The kinase-inducibledomain (KID) containsthe phosphorylatedSer-133 and mul- I~cBc~ tiple other phosphorylationsites. In Jun, the 8 MOTIFS ANKYRIN domain that is absent in v-Jun is indicated. In REL SIMILARITY STAT1 the DNA-bindingdomain has not been mapped. The regions of Src homology(SH2 and SH3) are indicated. In NF-KB the Rel homologydomain is shown, which is conserved in all other members of the family. The inhibitor of NF-~<B, IKB, is also shown. In NF-ATp the region of weak Rel similaritythat containsthe DNA-bindingdomain is indicated. potentiates transcriptional initiation remain only poorly un- this section we shall briefly review the representative derstood. For example, transcriptional activation by the examples of these strategies, which are summarized in extensively studied transcription factor CREB (cAMP re- Table 2. sponse element-binding protein) is dependent on regu- lated phosphorylation at Ser-133; this can be brought Activation in the Nucleus: MAP Kinase Pathways about independently by protein kinase A-, calmodulin-, or MAP kinases (MAPKs) are a family of protein.kinases nerve growth factor-induced kinases and is thus a conver- whose prototype members are the mammalian extracel- gence point for different signaling pathways (Gonzalez lular signal-regulated kinases ERK1 and ERK2 and the and Montminy, 1989; Sheng et al., 1991; Ginty et al., Saccharomyces cerevisiae pheromone-regulated kinases 1994). Ser-133 phosphorylation allows CREB to associate KSS1 and FUS3. MAPK phosphorylation sites contain the with a coactivator protein, CBP (Chrivia et al., 1993; Arias core sequence motif S/T-P. A number of nuclear transcrip- et al., 1994; Kwok et al., 1994); however, this does not tion factors have been identified as targets for MAPKs in appear to be sufficient for transcriptional activation, which metazoans, and MAPK signaling pathways provide rela- also involves neighboring glutamine-rich domains (Figure tively well-characterized examples of signaling cascades 1 ; for review see Lalli and Sassone-Corsi, 1994; see also with nuclear targets. Gonzalez and Montminy, 1989; Brindle et al., 1993; AI- The Ras/ERK MAPK Pathway berts et al., 1994a; Ferreri et al., 1994). In the case of Activation of receptor tyrosine kinases activates a signal- CREB, interaction with either specific cofactors like CBP ing cascade involving transient formation of ras-GTP and or the basal transcription machinery may be brought about activation of raf kinase at the membrane, followed by se- by a phosphorylation-induced conformational change in quential activation of MAPK kinase (MAPKK) and ERK1/ the protein (Gon zalez et al., 1991); phosphorylation-induced ERK2; only the latter enter the nucleus (for reviews see conformation changes may also be important in regulated Marshall, 1994; Leevers et al., 1994; Stokoe