Just as people head to the beaches for a well-deserved rest, accumulating evidence suggests that transcription factors take similar 'vacations' at the nuclear envelope. Recent studies indicate that the periphery of the nucleus provides a platform for sequestering transcription factors away from chromatin. Several transcriptional regulators, operating in different signal-transduction pathways, have been found to interact physically with components of the inner nuclear membrane. In general, this association seems to restrict access to their target genes and limit their transactivation or transrepression abilities. The mechanisms of inner nuclear membrane association are diverse, and include regulated associations with the nuclear lamina and integral membrane proteins. Together, these findings indicate that the inside of the nuclear envelope functions as a resting place for transcription factors and suggest a more direct role for the nuclear envelope in gene regulation than previously anticipated.

The proteasome-interacting protein Rad23 is a long-lived protein. Interaction between Rad23 and the proteasome is required for Rad23's functions in nucleotide excision repair and ubiquitin-dependent degradation. Here, we show that the ubiquitin-associated (UBA)-2 domain of yeast Rad23 is a cis-acting, transferable stabilization signal that protects Rad23 from proteasomal degradation. Disruption of the UBA2 domain converts Rad23 into a short-lived protein that is targeted for degradation through its N-terminal ubiquitin-like domain. UBA2-dependent stabilization is required for Rad23 function because a yeast strain expressing a mutant Rad23 that lacks a functional UBA2 domain shows increased sensitivity to UV light and, in the absence of Rpn10, severe growth defects. The C-terminal UBA domains of Dsk2, Ddi1, Ede1, and the human Rad23 homolog hHR23A have similar protective activities. Thus, the UBA2 domain of Rad23 is an evolutionarily conserved stabilization signal that allows Rad23 to interact with the proteasome without facing destruction.

The transcription factor Stp1 is endoproteolytically processed in response to extracellular amino acids by the plasma membrane SPS (Ssy1-Ptr3-Ssy5)-sensor. Processed Stp1, lacking a cytoplasmic retention motif, enters the nucleus and induces amino acid transporter gene expression. The SPS-sensor component Ssy5 is a chymotrypsin-like protease with a Pro-domain and a catalytic domain. The Pro-domain, required for protease maturation, is autolytically cleaved from the catalytic domain but remains associated, forming an inactive protease complex that binds Stp1. Stp1 is processed only after amino acid-induced signals cause the dissociation of the inhibitory Pro-domain. Our findings demonstrate that gene expression can be controlled by regulating the enzymatic activity of an intracellular endoprotease. A growing number of transcription factors are known to be maintained as latent cytoplasmic factors that require proteolytic processing prior to nuclear targeting. The sterol regulatory element-binding protein is released from membranes in two successive rounds by site-specific membrane-bound proteases in a process termed regulated intramembrane proteolysis (RIP) (Brown et al. 2000). Similarly, the Drosophila NFB factor Relish requires signal-activated and caspase-mediated processing before translocation to the nucleus (Stö ven et al. 2003). In yeast, transcription factors Spt23 and Mga2 are activated via their release from C-terminal membrane anchors by a process termed regulated ubiquitin/proteasome-dependent processing (RUP) (Hoppe et al. 2000). The control of transcription factor latency by proteolytic processing appears to be a useful mechanism for regulating gene expression.Central to understanding latent factor activation is the unambiguous identification of the responsible protease, and perhaps more importantly, defining how its proteolytic activity is regulated. In principle, metabolic signals could directly control the catalytic activity of a protease (enzymatic regulation), or control factor access to a constitutively active protease (substrate regulation). To date, only substrate regulation has been shown to play a role in the activation of latent factors in eukaryotic cells. This raises the question: Is the enzymatic control of protease activity mechanistically incompatible with signaling processes that regulate gene expression by mobilizing latent factors?Saccharomyces cerevisiae offers a few well-defined signaling pathways from the plasma membrane to the nucleus, many of which are involved in sensing nutrient availability and regulating nutrient uptake ( The SPS-sensor component Ssy5 has emerged as a candidate processing protease, since an improved sequence comparison algorithm identified weak homology with serine proteases (Abdel-Sater et al. 2004;Andréasson 2004;Poulsen et al. 2006). Consistently, Ssy5 exhibits several hallmarks of a protease, including apparent constitutive autoproteolysis of a Pro-domain, and a predicted catalytic serine residue has been shown to be required for autolysis and Stp1 pro...

Stp1 and Stp2 are homologous transcription factors in yeast that are synthesized as latent cytoplasmic precursors with NH2-terminal regulatory domains. In response to extracellular amino acids, the plasma membrane–localized Ssy1–Ptr3–Ssy5 (SPS) sensor endoproteolytically processes Stp1 and Stp2, an event that releases the regulatory domains. The processed forms of Stp1 and Stp2 efficiently target to the nucleus and bind promoters of amino acid permease genes. In this study, we report that Asi1 is an integral component of the inner nuclear membrane that maintains the latent characteristics of unprocessed Stp1 and Stp2. In cells lacking Asi1, full-length forms of Stp1 and Stp2 constitutively induce SPS sensor–regulated genes. The regulatory domains of Stp1 and Stp2 contain a conserved motif that confers Asi1-mediated control when fused to an unrelated DNA-binding protein. Our results indicate that latent precursor forms of Stp1 and Stp2 inefficiently enter the nucleus; however, once there, Asi1 restricts them from binding SPS sensor–regulated promoters. These findings reveal an unanticipated role of inner nuclear membrane proteins in controlling gene expression.