Abstract
BackgroundS100A5 is a calcium binding protein found in a small subset of amniote tissues. Little is known about the biological roles of S100A5, but it may be involved in inflammation and olfactory signaling. Previous work indicated that S100A5 displays antagonism between binding of Ca2+ and Cu2+ ions—one of the most commonly cited features of the protein. We set out to characterize the interplay between Ca2+ and Cu2+ binding by S100A5 using isothermal titration calorimetry (ITC), circular dichroism spectroscopy (CD), and analytical ultracentrifugation (AUC).ResultsWe found that human S100A5 is capable of binding both Cu2+ and Ca2+ ions simultaneously. The wildtype protein was extremely aggregation-prone in the presence of Cu2+ and Ca2+. A Cys-free version of S100A5, however, was not prone to precipitation or oligomerization. Mutation of the cysteines does not disrupt the binding of either Ca2+ or Cu2+ to S100A5. In the Cys-free background, we measured Ca2+ and Cu2+ binding in the presence and absence of the other metal using ITC. Saturating concentrations of Ca2+ or Cu2+ do not disrupt the binding of one another. Ca2+ and Cu2+ binding induce structural changes in S100A5, which are measurable using CD spectroscopy. We show via sedimentation velocity AUC that the wildtype protein is prone to the formation of soluble oligomers, which are not present in Cys-free samples.ConclusionsS100A5 can bind Ca2+ and Cu2+ ions simultaneously and independently. This observation is in direct contrast to previously-reported antagonism between binding of Cu2+ and Ca2+ ions. The previous result is likely due to metal-dependent aggregation. Little is known about the biology of S100A5, so an accurate understanding of the biochemistry is necessary to make informed biological hypotheses. Our observations suggest the possibility of independent biological functions for Cu2+ and Ca2+ binding by S100A5.
Highlights
S100A5 is a calcium binding protein found in a small subset of amniote tissues
S100A5 is a member of the calcium-binding S100 protein family
We establish that the Cysteine-containing (WT) protein is prone to the formation of high-ordered oligomers in solution, while the Cysteine-free variant is almost entirely dimeric. We suggest that this propensity for formation of large oligomeric species and precipitation under our experimental conditions may underlie the apparent antagonism observed in the original S100A5 report
Summary
S100A5 is a calcium binding protein found in a small subset of amniote tissues. Little is known about the biological roles of S100A5, but it may be involved in inflammation and olfactory signaling. S100A5 undergoes a notable conformational change upon calcium-binding, resulting in the rotation and extension of a helix [1]. This Ca2+driven exposure of a hydrophobic surface is the primary mode of signal transduction in the S100 proteins [3]. Expression of the protein has been observed in a small number of other tissues [10]. It is used as a bio-marker for several types of brain cancers and inflammatory disorders and appears to be involved in inflammation via activation of RAGE [2, 11, 12]. Genetic work on S100A5 has been minimal, which has limited our understanding of its biological roles
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
