Dr. Carl Parker Group

Division of Chemistry and Chemical Engineering

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The current research in the Parker lab at Caltech focuses on the molecular mechanisms cells use to respond to physiological stress such as heat shock.  We are interested in understanding how heat for example is transduced into a massive transcriptional activation response that results in the synthesis of heat shock proteins.  It is understood that the heat shock proteins serve as a molecular chaperone and allow the cell to survive the denaturing conditions imposed by the elevated growth temperatures.  It is not understood how the heat signal is transduced into a transcriptional response.

In Drosophila and mammals the transcription factor responsible for activating transcription of the heat shock genes is the heat shock transcription factor (HSF).  In normally growing cells the HSF exists as an inactive monomer that cannot bind DNA or activate transcription.   When cells are subjected to heat shock the factor is actively transported into the nucleus trimerizes (giving it high affinity DNA binding properties) and undergoes further modifications which allow it to activate transcription.

Recent work from the lab has demonstrated that the domain that regulates monomer to trimer transition also controls nuclear entry.  When this domain is deleted from the protein it spontaneously trimerizes with out heat shock and thus acquires DNA binding capabilities but cannot enter the nucleus. During early stages of Drosophila development the heat shock response cannot be induced. It is reasoned that the adverse effects on cell cycle and cell growth brought about by Hsp70 induction must outweigh the beneficial aspects of Hsp70 induction in the early embryo.  Although the Drosophila heat shock transcription factor (dHSF) is abundant in the early embryo it does not enter the nucleus in response to heat shock.   In older embryos and in cultured cells the factor is localized within the nucleus in an apparent trimeric structure that binds DNA with high affinity.  The domain responsible for nuclear localization upon stress resides between residues 390 and 420 of the dHSF.  Using that domain as bait in a yeast two-hybrid system we now report the identification and cloning of a nuclear transport protein Drosophila karyopherin-a3 (dKap-a3).  Biochemical methods demonstrate that the dKap-a3protein binds tightly to the NLS.  Furthermore the dKap-a3 protein does not associate with NLSs that contain point mutations which are not transported in vivo.  Nuclear docking studies also demonstrate specific nuclear targeting of the NLS substrate by dKap-a3.  Previous studies from other laboratories have demonstrated that early Drosophila embryos are refractory to heat shock as a result of dHSF nuclear exclusion.  We demonstrate that the early embryo is deficient in dKap-a3 protein until cycle 12.  From cycle 13 onward the transport factor is present and the dHSF is localized within the nucleus thus allowing the embryo to respond to heat shock.