Partch's Lab currently focuses on the proteins known to circadian timekeeping, and utilizes a range of
structural and
biophysical techniques in order to characterize the biological role of these proteins including
NMR spectroscopy and
X-ray crystallography. In 2020, the lab published a paper describing how the mammalian circadian protein PERIOD and its cognate kinase Casein Kinase 1 form a molecular switch to regulate PERIOD protein stability, and therefore circadian periodicity.
Role of SasA protein in cyanobacteria Previously, many models of cyanobacterial time keeping were based solely on the continuous phosphorylation of the Kai proteins (
KaiA,
KaiB, and
KaiC) with SasA and CikA providing only input-output signaling. These earlier dependent models relied solely on KaiC acting as the main component of the
circadian oscillator with KaiA being used to
phosphorylase Threonine and
Serine and KaiB being used for their subsequent
dephosphorylation. For these reactions to work, ATP is broken down to ADP to provide the necessary energy and phosphate groups necessary to power these reactions. Partch challenged this assumption by modeling the effect of SasA proteins in differing concentrations of KaiA, KaiB, and KaiC. It was found that SasA uses structural mimicry to help fold-switched KaiB bind to the KaiC
hexamer so that the nighttime repressive complex can be formed. This maintains the rhythmicity of the circadian oscillator during limiting concentrations of KaiB by allowing both of the hexamers to auto phosphorylate and dephosphorylate threonine and serine. Conversely, SasA proteins compete with KaiB proteins for the binding of the KaiC hexamer when the concentration of SasA exceeds that of KaiB. The competition between these proteins can be mitigated when the concentration of SasA is less than or equal to half of the concentration of KaiB. Lower concentrations of SasA allow for KaiB to bind to the KaiC hexamer solely; it does not need to compete for KaiC binding spots with SasA.
PERIOD proteins and CK1 Carrie Partch has made significant discoveries pertaining to PERIOD protein's role in regulating the circadian clock. PERIOD proteins,
Per1and
Per2, create large, multimeric complexes with the circadian repressors
CRY1 and
CRY2. These complexes directly bind to and inhibit the core circadian
transcription factor,
CLOCK:BMAL1. As PERIOD proteins are central components of our biological clock, the regulation of PER1 and PER2's expression, modification, and protein stability is especially important. Additionally,
casein Kinase 1 (CK1) phosphorylates both the
Degron region (initiates PER degradation) and the FASP region (antagonistically stabilizes PER). Partch discovered and characterized the activity of CK1 on its biological substrate in vivo. Particularly, her findings demonstrated that the CK1 tau mutation, which reduces the oscillation cycle to roughly 20 hours, amplifies the Degron activity of CK1 while diminishing the FASP activity. Additionally, she identified the molecular switch involving an anion
binding site in CK1 that regulates the
phosphorylation of functionally antagonistic sites in PERIOD proteins. Her research showed that mutations in period-altering
kinases differentially regulate the activation loop switch to produce expected variations in PER2 stability, laying the groundwork for comprehending and controlling CK1's impact on circadian rhythms. However, Partch contributed to the development of the formalized phosphoswitch model, compiling the previous research into a single model. The phosphoswitch model is a proposed regulatory mechanism for the stabilization and destabilization of the PERIOD protein in the mammalian circadian clock. This model explains the circadian sensitivity and phenotypic differences caused by mutations within the
PER2 protein at site 662 and site 478. A downstream mutation from a serine to a glycine at site 662 leads to a shorter period, underphosphorylation, and PER2 destabilization. Because of the resulting shorter period, the phosphoswitch model is a possible mechanism for Familial
Advanced Sleep Phase Syndrome (FASPS). The exact role of phosphorylation within the FASP region in the stabilization of PER2 is not yet known. == Awards ==