Nucleo-cytoplasmic trafficking of metal-regulatory transcription factor 1 is regulated by diverse stress signals

The metal-regulatory transcription factor 1 (MTF-1) is a key regulator of h eavy metal-induced transcription of metallothionein I and II and other gene s in mammals and other metazoans. Transcriptional activation of genes by MT F-1 is mediated through binding to metal-responsive elements of consensus T GCRCNC in the target gene promoters. In an attempt to further clarify the m echanisms by which certain external signals activate MTF-1 and in turn modu late gene transcription, we show here that human MTF-1 has a dual nuclear a nd cytoplasmic localization in response to diverse stress stimuli. MTF-1 co ntains a consensus nuclear localization signal located just N-terminal to t he first zinc finger that contributes to but is not essential for nuclear i mport. MTF-1 also harbors a leucine-rich, nuclear export signal. Under rest ing conditions, the nuclear export signal is required for cytoplasmic local ization of MTF-1 as indicated by mutational analysis and transfer to the he terologous green fluorescent protein. Export from the nucleus was inhibited by leptomycin B, suggesting the involvement of the nuclear export protein CRM1. Our results further show that in addition to the heavy metals zinc an d cadmium, heat shock, hydrogen peroxide, low extracellular pH (pH 6.0), in hibition of protein synthesis by cycloheximide, and serum induce nuclear ac cumulation of MTF-1. However, heavy metals alone land not the other stress conditions) induce a significant transcriptional response via metal-respons ive element promoter sequences, implying that nuclear import of MTF-1 is ne cessary but not sufficient for transcriptional activation. Possible roles f or nuclear import under nonmetal stress conditions are discussed.