Structure of the Calmodulin αII-Spectrin Complex Provides Insight into the Regulation of Cell Plasticity

Carol D. Cianci,Thomas A. Steitz,Miljan Simonovic,Jon S. Morrow,Zhushan Zhang

doi:10.1074/jbc.m604613200

Carol D. Cianci, Thomas A. Steitz + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m604613200

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Abstract

AlphaII-spectrin is a major cortical cytoskeletal protein contributing to membrane organization and integrity. The Ca2+-activated binding of calmodulin to an unstructured insert in the 11th repeat unit of alphaII-spectrin enhances the susceptibility of spectrin to calpain cleavage but abolishes its sensitivity to several caspases and to at least one bacterially derived pathologic protease. Other regulatory inputs including phosphorylation by c-Src also modulate the proteolytic susceptibility of alphaII-spectrin. These pathways, acting through spectrin, appear to control membrane plasticity and integrity in several cell types. To provide a structural basis for understanding these crucial biological events, we have solved the crystal structure of a complex between bovine calmodulin and the calmodulin-binding domain of human alphaII-spectrin (Protein Data Bank ID code 2FOT). The structure revealed that the entire calmodulin-spectrin-binding interface is hydrophobic in nature. The spectrin domain is also unique in folding into an amphiphilic helix once positioned within the calmodulin-binding groove. The structure of this complex provides insight into the mechanisms by which calmodulin, calpain, caspase, and tyrosine phosphorylation act on spectrin to regulate essential cellular processes.

Highlights

Spectrin ␣␤ heterodimers are organized into filamentous structures composed of two related subunits
Cleavage by calpain is controlled by phosphorylation of residue Tyr1176 by c-Src, a kinase that binds to the flanking SH3 domain in the ␣10 repeat unit [31]
These numerous hydrophobic interactions are reminiscent of those seen within a protein core, suggesting that complex formation is driven by an increase in solvent entropy

Summary

Introduction

Spectrin ␣␤ heterodimers are organized into filamentous structures composed of two related subunits. The structure of this complex revealed that the CaM-␣IIspec-binding interface is composed entirely of hydrophobic residues, in striking contrast to previously determined CaM-peptide structures. The crystal structure of the CaM-␣IIspec complex provides a compelling structural basis for understanding how calmodulin regulates the susceptibility of ␣II-spectrin to proteolysis and thereby other downstream cytoskeletal signals.

Results

Conclusion