ABSTRACT
BACKGROUND: The transcription factor NKX2-5 is crucial for heart development, and mutations in this gene have been implicated in diverse congenital heart diseases and conduction defects in mouse models and humans. Whether NKX2-5 mutations have a role in adult-onset heart disease is unknown. METHODS AND RESULTS: Mutation screening was performed in 220 probands with adult-onset dilated cardiomyopathy. Six NKX2-5 coding sequence variants were identified, including 3 nonsynonymous variants. A novel heterozygous mutation, I184M, located within the NKX2-5 homeodomain, was identified in 1 family. A subset of family members had congenital heart disease, but there was an unexpectedly high prevalence of dilated cardiomyopathy. Functional analysis of I184M in vitro demonstrated a striking increase in protein expression when transfected into COS-7 cells or HL-1 cardiomyocytes because of reduced degradation by the Ubiquitin-proteasome system. In functional assays, DNA-binding activity of I184M was reduced, resulting in impaired activation of target genes despite increased expression levels of mutant protein. CONCLUSIONS: Certain NKX2-5 homeodomain mutations show abnormal protein degradation via the Ubiquitin-proteasome system and partially impaired transcriptional activity. We propose that this class of mutation can impair heart development and mature heart function and contribute to NKX2-5-related cardiomyopathies with graded severity.
Subject(s)
Cardiomyopathies/genetics , Heart Defects, Congenital/genetics , Homeodomain Proteins/genetics , Homeodomain Proteins/metabolism , Transcription Factors/genetics , Transcription Factors/metabolism , Adolescent , Adult , Age of Onset , Amino Acid Sequence , Animals , Base Sequence , COS Cells , Cardiomyopathies/metabolism , Chlorocebus aethiops , Female , Heart Defects, Congenital/metabolism , Homeobox Protein Nkx-2.5 , Homeodomain Proteins/chemistry , Humans , Male , Middle Aged , Molecular Sequence Data , Mutation , Myocytes, Cardiac/metabolism , Pedigree , Proteolysis , Sequence Alignment , Transcription Factors/chemistry , Transcriptional Activation , Young AdultABSTRACT
In the trypsin superfamily of serine proteases, non-trypsin-like primary specificities have arisen in only two monophyletic descendent subbranches. We have recreated an ancestor to one of these subbranches (granzyme) using phylogenetic inference, gene synthesis, and protein expression. This ancestor has two unusual properties. First, it has broad primary specificity encompassing the entire repertoire of novel primary specificities found in its descendents. Second, unlike extant members that have narrow primary specificities, the ancestor exhibits tolerance to mutational changes in primary specificity-conferring residues-that is, structural plasticity. Molecular modeling and mutagenesis studies indicate that these unusual properties are due to a particularly wide substrate binding pocket. These two crucial properties of the ancestor not only distinguish it from its extant descendents but also from the trypsin-like proteases that preceded it. This indicates that a despecialization step, characterized by broad specificity and structural plasticity, underlies evolution of new primary specificities in this protease superfamily.
Subject(s)
Serine Endopeptidases/chemistry , Serine Endopeptidases/genetics , Serine Endopeptidases/metabolism , Amino Acid Sequence , Animals , Catalysis , Chymotrypsin/chemistry , Evolution, Molecular , Granzymes , Kinetics , Mice , Models, Molecular , Molecular Sequence Data , Mutagenesis, Site-Directed , Mutation , Phenylalanine/chemistry , Phylogeny , Plasmids/metabolism , Point Mutation , Protein Binding , Rats , Recombinant Proteins/metabolism , Sheep , Substrate Specificity , Time Factors , Trypsin/chemistry , Trypsin/pharmacologyABSTRACT
One of the promising methods of protein structure prediction involves the use of amino acid sequence-derived patterns. Here we report on the creation of non-degenerate motif descriptors derived through data mining of training sets of residues taken from the transmembrane-spanning segments of polytopic proteins. These residues correspond to short regions in which there is a deviation from the regular alpha-helical character (i.e. pi-helices, 3(10)-helices and kinks). A 'search engine' derived from these motif descriptors correctly identifies, and discriminates amongst instances of the above 'non-canonical' helical motifs contained in the SwissProt/TrEMBL database of protein primary structures. Our results suggest that deviations from alpha-helicity are encoded locally in sequence patterns only about 7-9 residues long and can be determined in silico directly from the amino acid sequence. Delineation of such variations in helical habit is critical to understanding the complex structure-function relationships of polytopic proteins and for drug discovery. The success of our current methodology foretells development of similar prediction tools capable of identifying other structural motifs from sequence alone. The method described here has been implemented and is available on the World Wide Web at http://cbcsrv.watson.ibm.com/Ttkw.html.
