Abstract
Helicases play a critical role in processes such as replication or recombination by unwinding double-stranded DNA; mutations of these genes can therefore have devastating biological consequences. In humans, mutations in genes of three members of the RecQ family helicases (blm, wrn, and recq4) give rise to three strikingly distinctive clinical phenotypes: Bloom syndrome, Werner syndrome, and Rothmund-Thomson syndrome, respectively. However, the molecular basis for these varying phenotypic outcomes is unclear, in part because a full mechanistic description of helicase activity is lacking. Because the helicase core domains are highly conserved, it has been postulated that functional differences among family members might be explained by significant differences in the N-terminal domains, but these domains are poorly characterized. To help fill this gap, we now describe bioinformatics, biochemical, and structural data for three vertebrate BLM proteins. We pair high resolution crystal structures with SAXS analysis to describe an internal, highly conserved sequence we term the dimerization helical bundle in N-terminal domain (DHBN). We show that, despite the N-terminal domain being loosely structured and potentially lacking a defined three-dimensional structure in general, the DHBN exists as a dimeric structure required for higher order oligomer assembly. Interestingly, the unwinding amplitude and rate decrease as BLM is assembled from dimer into hexamer, and also, the stable DHBN dimer can be dissociated upon ATP hydrolysis. Thus, the structural and biochemical characterizations of N-terminal domains will provide new insights into how the N-terminal domain affects the structural and functional organization of the full BLM molecule.
Highlights
APRIL 7, 2017 VOLUME 292 NUMBER 14 during cellular processes, such as replication, transcription, or repair [1, 2]
The RecQ carboxyl-terminal (RQC) domain is restricted to RecQ family members and is considered important for both the structural integrity of the protein and doublestranded DNA (dsDNA) binding; it might have a role in mediating interactions with other proteins [10, 11]
Bioinformatics analysis reveals that DHBN is the only highly conserved domain in N-terminal domains of the vertebrate BLM homologues—To probe whether the N-terminal domains are evolutionarily conserved in sequence and structure, we performed multiple-sequence alignment of most homologous proteins of BLM (BLMs) in the reference proteome database and constructed a phylogenetic tree
Summary
Helicases play a critical role in processes such as replication or recombination by unwinding double-stranded DNA; mutations of these genes can have devastating biological consequences. Because the helicase core domains are highly conserved, it has been postulated that functional differences among family members might be explained by significant differences in the N-terminal domains, but these domains are poorly characterized. To help fill this gap, we describe bioinformatics, biochemical, and structural data for three vertebrate BLM proteins. The unwinding activity of gBLM is regulated by the different oligomers, and the dimer is the basic unit to form a hexamer These findings shed new light on the enzymatic properties and structure of BLM protein and deepen our understanding of the molecular basis of the functional specificity of BLM in cells
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