4NDB

X-ray structure of a mutant (T61D) of calexcitin - a neuronal calcium-signalling protein

Summary for 4NDB

• Entry DOI: 10.2210/pdb4ndb/pdb
• Related: 2CCM 4NDC 4NDD
• Descriptor: Calexcitin, CALCIUM ION (3 entities in total)
• Functional Keywords: ef-hand, calcium-binding, neuron, signaling protein
• Biological source: Doryteuthis pealeii (longfin inshore squid)
• Total number of polymer chains: 2
• Total formula weight: 49938.03

Authors

Erskine, P.T.,Fokas, A.,Muriithi, C.,Razzall, E.,Bowyer, A.,Findlow, I.S.,Werner, J.M.,Wallace, B.A.,Wood, S.P.,Cooper, J.B. (deposition date: 2013-10-25, release date: 2014-10-29, Last modification date: 2023-09-20)

Primary citation

Erskine, P.T.,Fokas, A.,Muriithi, C.,Rehman, H.,Yates, L.A.,Bowyer, A.,Findlow, I.S.,Hagan, R.,Werner, J.M.,Miles, A.J.,Wallace, B.A.,Wells, S.A.,Wood, S.P.,Cooper, J.B.
X-ray, spectroscopic and normal-mode dynamics of calexcitin: structure-function studies of a neuronal calcium-signalling protein.
Acta Crystallogr.,Sect.D, 71:615-631, 2015

PubMed Abstract: The protein calexcitin was originally identified in molluscan photoreceptor neurons as a 20 kDa molecule which was up-regulated and phosphorylated following a Pavlovian conditioning protocol. Subsequent studies showed that calexcitin regulates the voltage-dependent potassium channel and the calcium-dependent potassium channel as well as causing the release of calcium ions from the endoplasmic reticulum (ER) by binding to the ryanodine receptor. A crystal structure of calexcitin from the squid Loligo pealei showed that the fold is similar to that of another signalling protein, calmodulin, the N- and C-terminal domains of which are known to separate upon calcium binding, allowing interactions with the target protein. Phosphorylation of calexcitin causes it to translocate to the cell membrane, where its effects on membrane excitability are exerted and, accordingly, L. pealei calexcitin contains two protein kinase C phosphorylation sites (Thr61 and Thr188). Thr-to-Asp mutations which mimic phosphorylation of the protein were introduced and crystal structures of the corresponding single and double mutants were determined, which suggest that the C-terminal phosphorylation site (Thr188) exerts the greatest effects on the protein structure. Extensive NMR studies were also conducted, which demonstrate that the wild-type protein predominantly adopts a more open conformation in solution than the crystallographic studies have indicated and, accordingly, normal-mode dynamic simulations suggest that it has considerably greater capacity for flexible motion than the X-ray studies had suggested. Like calmodulin, calexcitin consists of four EF-hand motifs, although only the first three EF-hands of calexcitin are involved in binding calcium ions; the C-terminal EF-hand lacks the appropriate amino acids. Hence, calexcitin possesses two functional EF-hands in close proximity in its N-terminal domain and one functional calcium site in its C-terminal domain. There is evidence that the protein has two markedly different affinities for calcium ions, the weaker of which is most likely to be associated with binding of calcium ions to the protein during neuronal excitation. In the current study, site-directed mutagenesis has been used to abolish each of the three calcium-binding sites of calexcitin, and these experiments suggest that it is the single calcium-binding site in the C-terminal domain of the protein which is likely to have a sensory role in the neuron.

PubMed: 25760610
DOI: 10.1107/S1399004714026704

Experimental method

X-RAY DIFFRACTION (2 Å)