Calcium-Binding Proteins with Disordered Structure and Their Role in Secretion, Storage, and Cellular Signaling

Abstract

Calcium is one of the most important second messengers and its intracellular signaling regulates many aspects of cell physiology. Calcium ions, like phosphate ions, are highly charged and thus are able to alter protein conformation upon binding; thereby they constitute key factors in signal transduction. One of the most common calcium-binding structural motifs is the EF-hand, a well-defined helix-loop-helix structural domain, present in many calcium-binding proteins (CBPs). Nonetheless, some CBPs contain non-canonical, disordered motifs, which usually bind calcium with high capacity and low affinity, and which represent a subset of proteins with specific functions, but these functions rarely involve signaling. When compared with phosphorylation-mediated signal transduction, the role of intrinsic disorder in calcium signaling is significantly less prominent and not direct. The list of known examples of intrinsically disordered CBPs is relatively short and the disorder in these examples seems to be linked to secretion and storage. Calcium-sensitive phosphatase calcineurin is an exception, but it represents an example of transient disorder, which is, nevertheless, vital to the functioning of this protein. The underlying reason for the different role of disordered proteins in the two main cellular signaling systems appears to be linked to the gradient of calcium concentration, present in all living cells.

Keywords: calcium binding proteins; calcium signaling; intrinsic disorder.

Figure 1

Structure and function of bacterial adenylate cyclase toxin (CyaA). (A) Structure of the C-terminal RTX domain (RD) of CyaA, with five blocks containing repeat-in-toxin (RTX) consensus. Variability of the motif can be observed in all blocks, RTX motifs in red frames, full consensus in red letters. (B) Left panel: calcium-bound RTX domain of bacterial toxin CyaA, β-roll structure, PDB_5CVW, right panel: the same structure from another perspective, visible narrow parallel β-roll. (C) Bacterial toxin with RTX domain acquires the structure upon Ca²⁺ binding. 1. Protein in bacterial cell: C-terminal domain is unstructured (red) in a low Ca²⁺ concentration. 2. Protein translocates into extracellular space through type 1 secretion system (T1SS): in a high concentration of calcium ions C-terminal domain acquires secondary structure (β-roll) 3. The toxin forms pores in mammalian cell membrane; part of the toxin (catalytic domain) translocates into the mammalian cell, binds calmodulin and causes detrimental increase in cellular cAMP.

Figure 2

Ca²⁺ binding by structured motifs of calmodulin and calreticulin. Calcium ions in green. (A) Calmodulin without calcium (well-structured), PDB_1CFC (B) Calcium-bound calmodulin, PDB_1UP5. The structure has changed, but remains well-ordered. (C) Calmodulin bound to a fragment of calcineurin PDB_2JZI. (D) Globular domain of calreticulin containing a single, high-affinity Ca²⁺ binding site, PDB_3POW.