The clinical severity of Osteogenesis Imperfecta (OI), also known as the brittle bone disease, relates to the extent of conformational changes in the collagen triple helix induced by Gly substitution mutations. The lingering question is why Gly substitutions at different locations of collagen cause different disruptions of the triple helix. Here, we describe markedly different conformational changes of the triple helix induced by two Gly substitution mutations placed only 12 residues apart. The effects of the Gly substitutions were characterized using a recombinant collagen fragment modeling the 63-residue segment of the ␣1 chain of type I collagen containing no Hyp (residues 877-939) obtained from Escherichia coli. Two Gly 3 Ser substitutions at Gly-901 and Gly-913 associated with, respectively, mild and severe OI variants were introduced by site-directed mutagenesis. Biophysical characterization and limited protease digestion experiments revealed that while the substitution at Gly-901 causes relatively minor destabilization of the triple helix, the substitution at Gly-913 induces large scale unfolding of an unstable region C-terminal to the mutation site. This extensive unfolding is caused by the intrinsic low stability of the C-terminal region of the helix and the mutation induced disruption of a set of salt bridges, which functions to lock this unstable region into the triple helical conformation. The extensive conformational changes associated with the loss of the salt bridges highlight the long range impact of the local interactions of triple helix and suggest a new mechanism by which OI mutations cause severe conformational damages in collagen.Considerable effort has been made to elucidate the mechanisms by which Gly substitution mutations of the collagen triple helix cause Osteogenesis Imperfecta (OI), 2 also known as brittle bone disease. The collagen triple helix consists of three polypeptide chains each in extended polyproline II conformation and with the characteristic (Gly-X-Y) n repeating amino acid sequence (1-3). The Gly at every third position is necessitated by the close packing of the helix; while the X and Y residues (where X and Y can be any amino acids) contribute directlytothestabilityofthetriplehelixandconferthesequencedependent properties of collagen (4). Missense mutations that replace the obligatory Gly by another amino acid residue in type I collagen, the major component of bones, are the most common cause of OI (5, 6). The triple helix domain of type I collagen is a heterotrimer composed of two ␣1 chains and one ␣2 chain each with more than 1000 amino acids in an uninterrupted (Gly-X-Y) n sequence (7). Nearly 800 Gly replacing mutations from both ␣1 and ␣2 chains have been linked to OI, yet, depending on the location and the identity of the Gly substitution, the clinical severity of OI varies from mild increase of bone fragility to the most severe type characterized by death at the prenatal stage (the Type II OI) (6). It remains unclear what molecular properties are related to the se...
Proponents of the neutral theory argue that evolution at the molecular level lagely reflects a process of random genetic drift of neutral mutations. Under this theory, levels of interspeific divergence and intapecfic polymorphism are expected to be correlated across dasses of nudeotide or amino acid sequences with different degrees of functional constraint, such as synonymous and replcement sites. Nudeotide sites with reduced polymorphbm should show comparably reduced levels of interspecific divergence. To examine this hypothesis, we have sequenced 32 and 12 copies of the glucose-6-phosphate dehydrogenase (G6pd) gene in Drosophila melanogaster and Drosophila sinulans, respectively. Both species exhibit similar levels of nucleotide polymorphism at synonymous sites. D. melanogaster shows two amino acid polymorphisms, one associated with the cosmopolitan aflozyme polymorphism and a second with an allozyme polymorphism endemic to European and North African populations. In contrast, D. simulans shows no replacement polymorphism. While synonymous divergence between specks is 10%, which is typical of other genes, there are 21 replacement differences. This level of amino acid sequence divergence, when contrasted with levels of amino acid polymorphism, silent polymorphism, and divergence, is in 10-fold excess over that expected under the neutral model of molecular evolution. We propose that this excess divergence reflects episodes of natural selecton on G6pd resulting in fixation of advantageous amino acid mutations in these two recentiy separated lineages.The extent that intra-and interspecific variation in DNA and amino acid sequences reflects a process of adaptation or is simply molecular noise remains one ofthe enduring questions in evolutionary biology. The alternative model to simple adaptive change, or neutral theory, assumes the action of natural selection at the molecular level, but only acting in a purifying fashion. Thus, most amino acid mutations are assumed to be eliminated by natural selection. A small minority, those satisfying the criterion that they minimally disrupt protein function and thus confer no fitness loss on the individual, face eventual extinction or fixation through a stochastic process whose transition time depends on population size. Insofar as observed patterns of DNA sequence divergence between species and standing levels of molecular polymorphism within species are concerned, the theory and its proponents dismiss a significant positive role for selection, either in adaptive substitution or in the maintenance of molecular polymorphism (1, 2).Two types of quantitative analyses have been used to examine the hypothesis that protein sequence divergence at the interspecific level is neutral. One argument for adaptive amino acid change uses the index of dispersion to infer heterogeneous rates of substitution across phylogenies (3).The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.