Multiple novel signals mediate thyroid hormone receptor nuclear import and export

Thyroid hormone receptor (TR) is a member of the nuclear receptor superfamily that shuttles between the cytosol and nucleus. The fine balance between nuclear import and export of TR has emerged as a critical control point for modulating thyroid hormone-responsive gene expression; however, sequence motifs of TR that mediate shuttling are not fully defined. Here, we characterized multiple signals that direct TR shuttling. Along with the known nuclear localization signal in the hinge domain, we identified a novel nuclear localization signal in the A/B domain of thyroid hormone receptor α1 that is absent in thyroid hormone receptor β1 and inactive in the oncoprotein v-ErbA. Our prior studies showed that thyroid hormone receptor α1 exits the nucleus through two pathways, one dependent on the export factor CRM1 and the other CRM1-independent. Here, we identified three novel CRM1-independent nuclear export signal (NES) motifs in the ligand-binding domain as follows: a highly conserved NES in helix 12 (NES-H12) and two additional NES sequences spanning helix 3 and helix 6, respectively. Mutations predicted to disrupt the α-helical structure resulted in a significant decrease in NES-H12 activity. The high degree of conservation of helix 12 suggests that this region may function as a key NES in other nuclear receptors. Furthermore, our mutagenesis studies on NES-H12 suggest that altered shuttling of thyroid hormone receptor β1 may be a contributing factor in resistance to thyroid hormone syndrome. Taken together, our findings provide a detailed mechanistic understanding of the multiple signals that work together to regulate TR shuttling and transcriptional activity, and they provide important insights into nuclear receptor function in general.

Recruitment of the oncoprotein v-ErbA to aggresomes

Aggresome formation, a cellular response to misfolded protein aggregates, is linked to cancer and neurodegenerative disorders. Previously we showed that Gag-v-ErbA (v-ErbA), a retroviral variant of the thyroid hormone receptor (TRα1), accumulates in and sequesters TRα1 into cytoplasmic foci. Here, we show that foci represent v-ErbA targeting to aggresomes. v-ErbA colocalizes with aggresomal markers, proteasomes, hsp70, HDAC6, and mitochondria. Foci have hallmark characteristics of aggresomes: formation is microtubule-dependent, accelerated by proteasome inhibitors, and they disrupt intermediate filaments. Proteasome-mediated degradation is critical for clearance of v-ErbA and T(3)-dependent TRα1 clearance. Our studies highlight v-ErbA's complex mode of action: the oncoprotein is highly mobile and trafficks between the nucleus, cytoplasm, and aggresome, carrying out distinct activities within each compartment. Dynamic trafficking to aggresomes contributes to the dominant negative activity of v-ErbA and may be enhanced by the viral Gag sequence. These studies provide insight into novel modes of oncogenesis across multiple cellular compartments.

Processing of human cytomegalovirus UL37 mutant glycoproteins in the endoplasmic reticulum lumen prior to mitochondrial importation

The human cytomegalovirus (HCMV) UL37 glycoprotein (gpUL37) is internally cleaved and its products divergently traffic to mitochondria or are retained in the secretory pathway. To define the requirements for gpUL37 cleavage, residues -1 and -3 of the consensus endoplasmic reticulum (ER) signal peptidase I site within exon 3 (UL37x3) were replaced by bulky tyrosines (gpUL37 cleavage site mutant I). Internal cleavage of this UL37x3 mutant was inhibited, verifying usage of the consensus site at amino acids (aa) 193/194. The full-length mitochondrial species of gpUL37 cleavage site mutant I was N glycosylated and endoglycosidase H sensitive, indicating that ER translocation and processing took place prior to its mitochondrial importation. Moreover, these results suggest that internal cleavage of gpUL37 is not necessary for its N glycosylation. Partial deletion or disruption of the UL37 hydrophobic core immediately upstream of the cleavage site resulted in decreased protein abundance, suggesting that the UL37x3 hydrophobic alpha-helix contributes to either correct folding or stability of gpUL37. Insertion of the UL37x3 hydrophobic core and cleavage site into pUL37(M), a splice variant of gpUL37 which lacks these sequences and is neither proteolytically cleaved nor N glycosylated, resulted in its internal cleavage and N glycosylation. Its NH(2)-terminal fragment, pUL37(M-NH2), was detected more abundantly in mitochondria, while its N-glycosylated C-terminal fragment, gpUL37(M-COOH), was detected predominantly in the ER in a manner analogous to that of gpUL37 cleavage products. These results indicate that UL37x3 aa 178 to 205 are prerequisite for gpUL37 internal cleavage and alter UL37 protein topology allowing N glycosylation of its C-terminal sequences. In contrast, the NH(2)-terminal UL37x1 hydrophobic leader, present in pUL37x1, pUL37(M), and gpUL37, is not cleaved from mature UL37 protein, retaining a membrane anchor for UL37 isoforms during trafficking. Taken together, these results suggest that HCMV gpUL37 undergoes sequential trafficking, during which it is ER translocated, processed, and then mitochondrially imported.

Internal cleavage of the human cytomegalovirus UL37 immediate-early glycoprotein and divergent trafficking of its proteolytic fragments

The human cytomegalovirus UL37 gene encodes at least three isoforms, which share N-terminal UL37 exon 1 (UL37x1) sequences. UL37 proteins traffic dually into the endoplasmic reticulum (ER) and to mitochondria. Trafficking of the UL37 glycoprotein (gpUL37) in relation to its post-translational processing was investigated. gpUL37 is internally cleaved in the ER and its products traffic differentially. Its C-terminal fragment (UL37(COOH)) is ER-localized and N-glycosylated. Unlike conventional ER signal sequences, its N-terminal fragment is stable and traffics to mitochondria. Inhibition of N-glycosylation did not block pUL37 cleavage and dramatically decreased the levels of but not of UL37(COOH). pUL37(M), which differs from gpUL37 by the lack of residues 178-262 and hence the UL37x3 consensus signal peptidase cleavage site, traffics into the ER and mitochondria, but is neither cleaved nor N-glycosylated. This finding of a relationship between ER processing and mitochondrial importation of UL37 proteins is unique for herpesvirus proteins.

Dual targeting of the human cytomegalovirus UL37 exon 1 protein during permissive infection

The human cytomegalovirus (HCMV) UL37 immediate-early (IE) gene minimally encodes three protein isoforms that share NH(2)-terminal sequences. The predominant UL37 isoform detected during HCMV infection was the UL37 exon 1 protein (pUL37x1), which was produced from IE and, more abundantly, through late times of infection. pUL37x1 was localized in both the endoplasmic reticulum (ER) and mitochondria in infected cells. To determine which UL37x1 NH(2)-terminal residues serve as ER and mitochondrial targeting signals, we examined the subcellular localization of two deletion mutants. pUL37x1Delta2-23, which lacks the hydrophobic leader, is neither translocated into the ER nor imported mitochondrially; conversely, pUL37x1Delta23-34, lacking the juxtaposed basic residues, was translocated into the ER but only imported weakly into mitochondria. These studies show for the first time the temporal production and localization of pUL37x1 during HCMV infection. The trafficking patterns of mutants suggest that the pUL37x1 targeting signal to ER and mitochondria is bipartite.