Gene duplication results in extra copies of genes that can be sub-functionalized on structural and/or regulatory levels. Multiple paralogs are expressed in teleosts for troponin (Tn) components of the contractile unit (TnC, TnI and TnT), likely to maximize survival in different environmental conditions. The evolution of Tn subunits can be used as a model for understanding the variation in contractile function in ectotherms. The studies in this dissertation integrate evolutionary analysis with structural information to expand upon the knowledge of Tn function across phylogeny. Multiple parameters of cardiac structure and function were determined in vivo in the adult zebrafish, using high-resolution echocardiography, to accurately characterize the responses of this teleost model to both acute and chronic temperature perturbations. Cardiac output was modulated primarily by heart rate in response to acute temperature changes. With cold acclimation, a decreased E/A ratio suggests an increased reliance on atrial contraction for ventricular filling.The evolutionary history and sub-functionalization of the cardiac-specific TnC1 genes were characterized on both regulatory and structural levels. Three paralogs of TnC exist in fish, two of which are homologous to mammalian TnC1/cTnC. The TnC1 paralogs are likely the result of a tandem gene duplication that occurred in the common ancestor of the teleosts. In both zebrafish and trout hearts, TnC1 paralogs display temperature and chamber specific patterns in their usage of mRNA transcripts. While the zebrafish TnC1 paralogs have minimal variation in structure based on homology models, TnC1b has a higher Ca2+ affinity relative to TnC1a as measured by isothermal titration calorimetry. Variation in the apparent affinity of zebrafish TnC1 paralogs for Ca2+ results from dynamic conformational flexibility changes rather than from the direct interaction of site II with Ca2+. Finally, the inter-related roles of regulatory and structural sub-functionalization that guide the co-evolution of interacting proteins in the Tn complex were explored. Transcriptional expression patterns predict various TnC/TnI complexes exist in the zebrafish heart with differential interaction strengths between the N-TnC and TnI switch region. Domain-specific divergent selection pressures and interaction energies suggest that substitutions in the TnI switch region are crucial to modifying TnI/TnC function to maintain cardiac contraction with temperature changes. Through these studies, the interacting proteins of the Tn complex have been established as an important model of the functional divergence of paralogs in the adaptive evolution of the teleost cardiac contractile element.
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Thesis advisor: Tibbits, Glen
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