Biomedical subjects

Samita Bhattacharya

Publications and source records attributed to Samita Bhattacharya.

3 recordsLinked to original sources

PubMed2006

Stat1 and SUMO modification.

Many proteins are known to undergo small ubiquitin-related modifier (SUMO) modification by an E1-, E2-, and E3-dependent ligation process. Recognition that protein inhibitor of activated signal transducers and activators of transcription (STATs) (PIAS) proteins are SUMO E3 ligases raised the possibility that STATs may also be regulated by SUMO modification. Consistent with this possibility, a SUMO-ylation consensus site (PsiKxE; Psi indicates hydrophobic residue, and x indicates any residue) was identified in Stat1 (ie, (702)IKTE(705)), but not in other STATs. Biochemical analysis confirmed that Stat1 K(703) could be SUMO modified in vitro. Mutation of this critical lysine (ie, Stat1(K703R)) yielded a protein that, when expressed in Stat1(-/-) mouse embryonic fibroblasts (MEFs), exhibited enhanced DNA binding and nuclear retention. This was associated with modest changes in transcriptional and antiviral activity. However, mutation of the second critical residue in the SUMO consensus site, E(705) (ie, Stat1(E705A)), yielded a protein with wild-type DNA binding, nuclear retention, and transcriptional and antiviral activity. Similar observations were made when these mutants were expressed in primary Stat1(-/-) macrophages. These observations suggest that although Stat1 can uniquely be SUMO-ylated in vitro, this modification is unlikely to play an important role in regulating Stat1 activity in vivo.

Animals↗

PubMed2006

The allosteric enhancer PD81,723 increases chimaeric A1/A2A adenosine receptor coupling with Gs.

PD81,723 {(2-amino-4,5-dimethyl-3-thienyl)-[3-(trifluromethyl)-phenyl]methanone} is a selective allosteric enhancer of the G(i)-coupled A1 AR (adenosine receptor) that is without effect on G(s)-coupled A2A ARs. PD81,723 elicits a decrease in the dissociation kinetics of A1 AR agonist radioligands and an increase in functional agonist potency. In the present study, we sought to determine whether enhancer sensitivity is dependent on coupling domains or G-protein specificity of the A1 AR. Using six chimaeric A1/A2A ARs, we show that the allosteric effect of PD81,723 is maintained in a chimaera in which the predominant G-protein-coupling domain of the A1 receptor, the 3ICL (third intracellular loop), is replaced with A2A sequence. These chimaeric receptors are dually coupled with G(s) and G(i), and PD81,723 increases the potency of N6-cyclopentyladenosine to augment cAMP accumulation with or without pretreatment of cells with pertussis toxin. Thus PD81,723 has similar functional effects on chimaeric receptors with A1 transmembrane sequences that couple with G(i) or G(s). This is the first demonstration that an allosteric regulator can function in the context of a switch in G-protein-coupling specificity. There is no enhancement by PD81,723 of G(i)-coupled A2A chimaeric receptors with A1 sequence replacing A2A sequence in the 3ICL. The results suggest that the recognition site for PD81,723 is on the A1 receptor and that the enhancer acts to directly stabilize the receptor to a conformational state capable of coupling with G(i) or G(s).

Adenosine↗

PubMed2003

Regulation of Stat3 nuclear export.

Stat3 is the most pleiotropic member of the signal transducer and activator of transcription (STAT) family of transcription factors and mediates pivotal responses for the cytokine family. In resting cells, STATs, including Stat3, reside largely in the cytoplasm. Upon cytokine stimulation, they rapidly translocate to the nucleus, where they promote the expression of target genes. During the subsequent period of signal decay they are re-exported back to the cytoplasm in preparation for the next round of signaling. This process of nuclear export can be blocked by the fungal toxin leptomycin B (LMB). In contrast to what appears to be the case for Stat1, LMB treatment not only blocks the poststimulation export of Stat3 from the nucleus back to the cytoplasm, but also promotes the nuclear accumulation of Stat3 in resting cells. Remarkably, the LMB-dependent nuclear accumulation of Stat3 in resting cells is independent of tyrosine phosphorylation, highlighting the existence of a "basal" signaling pathway. Subsequent studies identified three nuclear export signal (NES) elements. Two of these elements, Stat3(306-318) and Stat3(404-414), corresponded to those recently identified in Stat1, and a third, Stat3(524-535), is novel. Stat3(306-318) appears to be important in the rapid nuclear export seen after stimulation (poststimulation export), whereas the Stat3(404-414) and Stat3(524-535) play a more important role in regulating basal nuclear export. In summary, these studies indicate that the process of Stat3 nuclear export is dependent on multiple NES elements.

Active Transport, Cell Nucleus↗