Office: 11C Schlundt Annex
117 Schweitzer Hall
University of Missouri
Columbia, MO 65211
|BS||Lafayette College||Easton, Penn.||Chemistry|
|PhD||University of Wisconsin||Madison, Wis.||Biochemistry|
Ca2+ influences eukaryotic signal transduction via interactions with a host of Ca2+-binding proteins. Many of these possess a Ca2+-binding motif called the "EF-hand". Despite a general similarity, individual EF-hands vary in their affinity for Ca2+ and Mg2+. The basis for the variation is poorly understood. We are exploring the issue in the parvalbumins — small vertebrate-specific proteins harboring two Ca2+-binding sites.
The parvalbumin family includes α and β sub-lineages. Mammals express one isoform from each lineage. Interestingly, both are expressed in the sensory cells of the auditory organ (the "organ of Corti") — α in the inner hair cells (IHCs), β in the outer hair cells (OHCs). Parvalbumins are believed to serve as cytosolic Ca2+ buffers. It is not clear why the IHC and OHC would recruit distinct isoforms. The answer may reside in the disparate Ca2+-binding properties of α and β.
The Ca2+-binding sites in rat α behave almost identically in titrations with Ca2+ or Mg2+, exhibiting very high affinity for Ca2+ and substantial affinity for Mg2+. By contrast, the binding sites in rat β are nonequivalent and exhibit reduced affinity for divalent ions. These differences between rat α and β are especially intriguing in light of their 50 percent sequence identity. We are analyzing these two parvalbumins, and select site-specific variants, by titration calorimetry, differential scanning calorimetry, x-ray crystallography (with Dr. J.J. Tanner, Dept. of Chemistry), and NMR spectroscopy.
Together with researchers at Washington University in St. Louis, we are also characterizing two other proteins present in the organ of Corti (OC). OCP1 and OCP2 were discovered in 1980. Although they are the most abundant proteins in the OC, their physiological functions are unknown. They are co-expressed by the cells comprising the epithelial gap-junction system (EGJS). Having no vasculature, the OC relies heavily on the EGJS for transport of nutrients, metabolites, and signaling molecules. OCP1 and OCP2 may assist in the regulation of gap-junction activity.
OCP2 displays extreme homology to Skp1, an essential subunit in a class of ubiquitin-protein ligases known as SCF complexes. Various proteins interact with Skp1 through a sequence motif called the F-box. These "F-box proteins" also associate with specific target proteins, positioning them for modification by the ubiquitination machinery. Ubiquitination can trigger a variety of biological consequences, including proteolytic destruction.
OCP1 is an F-box protein. In accordance with prediction, it associates tightly with OCP2. The biological target protein of the putative SCF complex containing OCP1 and OCP2 is presently unknown. In vitro, OCP1 associates with connexin 26, the gene product most commonly associated with hereditary deafness disorders. Whether this interaction has any physiological relevance is currently under investigation.
Novak Kujundžić R, Steffens WL, Brewer JM, Henzl MT, Ragland WL. Characterization of avian thymic hormone and chicken parvalbumin 3 target cells. Int Immunopharmacol. 2013 Feb;15(2):282-8. doi: 10.1016/j.intimp.2012.12.013. Epub 2013 Jan 16.
Henzl MT, Markus LA, Davis ME, McMillan AT. Simultaneous addition of two ligands: A potential strategy for estimating divalent ion affinities in EF-hand proteins by isothermal titration calorimetry. Methods. 2013 Mar 1;59(3):336-48. doi: 10.1016/j.ymeth.2012.12.003. Epub 2012 Dec 22.
Henzl MT, Sirianni AG, Wycoff WG, Tan A, Tanner JJ. Solution structures of polcalcin Phl p 7 in three ligation states: Apo-, hemi-Mg2+-bound, and fully Ca2+-bound. Proteins. 2013 Feb;81(2):300-15. doi: 10.1002/prot.24186. Epub 2012 Nov 12.
Srivastava D, Singh RK, Moxley MA, Henzl MT, Becker DF, Tanner JJ. The three-dimensional structural basis of type II hyperprolinemia. J Mol Biol. 2012 Jul 13;420(3):176-89. doi: 10.1016/j.jmb.2012.04.010. Epub 2012 Apr 16.
Henzl MT, Reed MA, Tan A. Heightened stability of polcalcin Phl p 7 is correlated with strategic placement of apolar residues. Biophys Chem. 2011 Nov;159(1):110-9. doi: 10.1016/j.bpc.2011.05.015. Epub 2011 May 26.