In this paper, we report differential scanning calorimetry studies of the temperature and molar mass dependences for the primary and secondary crystallization behavior of bisphenol A polycarbonate (BAPC). The molar mass dependence of the crystallization rate is found to be much stronger during primary than during secondary crystallization, confirming our earlier claims that primary and secondary processes occur by significantly different mechanisms. Investigations of the secondary crystallization process suggest the existence of a crossover phenomenon from secondary crystal formation at low temperatures to isothermal lamellar thickening at high temperatures. While the results of our low-temperature studies of BAPC provide further support for the model developed in a previous publication on poly(arylene ether ether ketone), evidences from atomic force microscopy and calorimetry of isothermal lamellar thickening above the crossover temperature lead us to anticipate a more unified view of polymer crystallization.

The androgen receptor (AR) has critical functions as a transcription factor in both normal and cancer cells, but the specific mechanisms that regulate its nuclear localization are not well defined. We found that an AR mutation commonly reported in prostate cancer generates an androgen-independent gain of function for nuclear import. The substitution, Thr877Ala, is within the ligand-binding domain, but the nuclear import gain of function is mediated by the bipartite nuclear localization signal (NLS) spanning the DNA-binding domain (DBD) and hinge region. Bipartite NLS activity depends on the structure provided by the DBD, and protein interactions with the bipartite NLS are repressed by the hinge region. The bipartite NLS is recognized by importin 7, a nuclear import receptor for several proteins. Importin 7 binding to AR, however, inhibits import by shielding the bipartite NLS. Androgen binding relieves the inhibition by inducing a switch that promotes exchange of importin 7 for karyopherin alpha import receptors. Importin 7 contributes to the regulation of AR import by restraining import until androgen is detected in the cytoplasm. N uclear import of proteins is mediated by cis-acting nuclear localization signals (NLSs) that usually contain either one or two clusters of basic amino acids (1). Import signals similar to the monopartite NLS in simian virus 40 (SV40) (PKKKRRV) and the bipartite NLS in nucleoplasmin (KRPAATKKAGQAKKKK) have been identified in hundreds of proteins and provide the specificity for import through interactions with nuclear import receptors (1). Three steps common to all NLS and transport receptor-mediated pathways are (i) receptor recognition of the NLS, (ii) translocation of the NLS-receptor complex through the nuclear pore complex (NPC), and (iii) dissociation of the NLS-receptor complex in the nucleoplasm (2). After release of the NLS-containing protein into the nucleoplasm, import receptors are exported to the cytoplasm for a new round of protein import.The import receptors that recognize NLSs in proteins are phylogenetically conserved. Within a species, import receptors are either members of the importin ␤ superfamily, which also includes structurally related export receptors (3) or the importin-␣ family of import receptors (4). The latter are termed karyopherin ␣ (KPNA) proteins. Importin ␤ superfamily members that mediate import are 95-to 125-kDa polypeptides composed of HEAT repeats, which were first recognized in Huntingtin, elongation factor 3, the PR65/A subunit of protein phosphatase 2A, and the lipid kinase TOR and can bind directly to NLSs. Importin ␤ proteins then undergo transient binding to nucleoporins during translocation through the NPC (5). Certain HEAT repeats within importin ␤ family members form a domain that can bind RanGTP, an interaction that promotes NLS cargo release from these receptors within the nucleus (6). KPNA proteins are encoded by seven genes in humans (7). They are polypeptides of ϳ60 kDa in size that are composed of Armadillo (ARM) repeats, which bin...

Nuclear receptors (NRs) comprise a superfamily of ligand-regulated transcription factors implicated in a host of physiological processes, including development, differentiation, and proliferation. Translated in the cytoplasm, NRs must undergo import into the nucleus in order to modulate transcription of target genes in response to cognate hormones. NRs also undergo export from the nucleus, and there is emerging evidence that NR nucleocytoplasmic shuttling contributes to their regulation. Nucleocytoplasmic shuttling may provide a nexus for crosstalk between NRs and kinase pathways. Here we review some of the key studies on nuclear import and export of steroid hormone receptors within the NR superfamily, address some of the challenges in experimental dissection of NR transport and discuss recent findings linking specific kinase pathways to NR export.

The androgen receptor undergoes nuclear import in response to ligand, but the mechanism by which it undergoes nuclear export is poorly understood. We developed a permeabilized cell assay to characterize nuclear export of the androgen receptor in LNCaP prostate cancer cells. We found that nuclear export of endogenous androgen receptor can be stimulated by short double-stranded DNA oligonucleotides. This androgen receptor export pathway is dependent on ATP hydrolysis and is enhanced by phosphatase inhibition with okadaic acid. Fluorescence recovery after photobleaching in permeabilized cells, under the conditions that stimulate androgen receptor export, suggested that double-stranded DNA-dependent export does not simply reflect the relief of a nuclear retention mechanism. A radiolabeled androgen was used to show that the androgen receptor remains ligand-bound during translocation through the nuclear pore complex. A specific inhibitor to the DNA-dependent protein kinase, NU7026, inhibits androgen receptor export and phosphorylation. In living cells, NU7026 treatment increases androgen-dependent transcription from endogenous genes that are regulated by androgen receptor. We suggest that DNA-dependent protein kinase phosphorylation of the androgen receptor, or an interacting component, helps target the androgen receptor for export from the nucleus.

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