MBE Advance Access published online on October 5, 2009
Molecular Biology and Evolution, doi:10.1093/molbev/msp236
Research Article |
Isoform Divergence of the Filamin Family of Proteins
1 Department of Biochemistry and Biophysics
2 Department of Cell and Developmental Biology
3 Structural Bioinformatics Core Facility, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
4 Office of Intramural Training and Education, National Institute of Health, Bethesda, MD 20892
* To whom correspondence should be addressed. Nikolay V. Dokholyan; Department of Biochemistry and Biophysics, University of North Carolina at Chapel Hill, School of Medicine, Campus Box 7260, Chapel Hill, NC 27599; Phone: (919)966-3137; Email: dokh{at}med.unc.edu
Received for publication February 18, 2009. Revision received July 30, 2009. Accepted for publication September 22, 2009.
The vertebrate filamin family (A, B and C) is part of the spectrin family of actin cross-linking proteins. Family members share high sequence similarity (>64%) and have both common and isoform distinct functionality. To identify the basis for isoform specific functionality, we perform an evolutionary trace of chordate filamin at the granularity of single residues. Our trace methodology is constrained to focus on neo-functionality by requiring that one isoform remain the ancestral type while at least one isoform has an accepted mutation. We call divergence meeting these characteristics "class-distinctive." To obtain a temporal and spatial context for class-distinctive residues, we derive an all-atom model of full-length filamin A by homology modeling and joining individual domains. We map onto our model both conserved and class-distinctive residues along with the period (Teleostei, Amphibian, Mammalian) in which they diverged. Our phylogenetic analysis suggests that filamins diverged from a common ancestral gene between urochordate and vertebrate lineages. Filamins also diverged the most just after gene duplication, in the Teleostei period, with filamin C remaining closest to ancestral filamin. At the residue level, domains with well-characterized interfaces, IgFLN 17 and IgFLN 21, have diverged in potentially critical residues in their adhesion protein binding interfaces, signifying that isoforms may bind or regulate ligand binding differentially. Similarly, isoform divergence in a region associated with F-actin binding regulation suggests that isoforms differentially regulate F-actin binding. In addition, we observe some class-distinctive residues in the vicinity of missense mutations that cause filamin A and B associated skeletal disorders. Our analysis, utilizing both spatial and temporal granularity, has identified potentially important residues responsible for vertebrate filamin isoform specific divergence – significantly in regions where few binding partners have been discovered to date – and suggests yet to be discovered filamin binding partners, and isoform specific differential regulation with these binding partners.