Recoverin

Calcium-free Recoverin

Calcium-bound Recoverin

Biology

Calcium-myristoyl switch

The most prominent feature of recoverin is the calcium-myristoyl switch. Myristoryl group is normally sequestered away in the protein's hydrophobic interior binding pocket. When the protein binds calcium ions, change in conformation brings out myristoyl group from the binding pocket so that the myristoyl group can interact with the target or the protein can translocate to a different region.

This type of calcium-myristoyl (or other acyl group) switch arose early in evolution of eukaryotes. The key residues that allow the conformation change is conserved in many such proteins from yeast to humans. Also, the key residues that make up the binding pocket of myristoyl group in the calcium-free state is also invariant.

Rhodopsin, Recoverin and Vision

Rhodopsin is a light receptor molecule. When a photon strikes rhodopsin, rhodopsin changes its conformation, and a series of molecular signaling events take place. The end result of all the molecular event is the sending of a neural signal to the brain to report that a photon has been received.

Eyes of mammals have various ways of adapting to many different light levels. One is by adjusting the size of the iris. The other is by tagging phosphate groups to rhodopsin so that rhodopsin is less likely to initiate the signaling cascade. Phosphates are attached by rhodopsin kinase (RK).

Recoverin is a calcium-binding protein with a relative molecular weight of 23 K. Recoverin is involved in the recovery phase of visual excitation and in adaptation to the background light. Calcium-bound form of recoverin slows down the activity of RK, thereby prolonging the light sensitivity of rhodopsin.

Interesting to note is the location of recoverin molecules. When recoverin is not bound to calcium, it stays in cytosol. This is probably because the myristoyl group is hidden inside the protein. When recoverin binds calcium, it migrates to the disc membrane and embeds itself into the membrane using the myristoyl group as the anchor.

Structure of Calcium-free Recoverin

Recoverin is composed of 11 alpha helices (named A to K) and 2 pairs of short beta sheets. Four pairs of the helices form EF-hand-like helix-loop-helix motifs. Overall, the EF-hand-like motifs are linked in a linear fashion, as compared to the two-domain dumbbell shape of calmodulin, which also has four EF-hand motifs.

Myristoyl group is attached to the N-terminal glycine. It is in an extended conformation. It is embedded deep into the protein, and is stabilized by five helices that provide hydrophobic and aromatic residues for the binding pocket. Helices B, C, E and F provide walls of the pocket, while helix A provide a "lid" for the binding pocket.

Structure of Calcium-bound Recoverin

The first notable change is the location of myristoyl group. Compared to the tightly tucked-in conformation of myristoyl group in the calcium-free state, calcium-bound state shows a remarkable change in conformation that brings out the myristoyl group to the exterior of the protein.

This change is brought on by three key changes in conformation when calcium binds EF-2 and EF-3. C-terminal domain does not change much when calcium binds. First, the orientation of the helices in the EF-1 changes to the open conformation (helices become perpendicular to each other) without binding calcium, much like classical calcium-bound EF-hand motif. Second, changes in the conformation of EF-2 and EF-3 when bound to calcium ion brings about the rotation at Gly96. This rotation results in the two domains rotating 45 degrees relative to each other when compared to calcium-free recoverin. Third, the binding of calcium brings about a rotation at Gly42.

All these changes lead to two important events. One, the hydrophobic pocket that provided a resting place for the myristoyl group is no longer present. Thus, the myristoyl group is free to move out. Two, the N-terminal end of recoverin swings itself out of the binding pocket. This results in physically pulling on myristoyl group, and subsequent ejection of the acyl group.

All these remarkable changes lead to the activation of recoverin. Myristoyl group can now look for its target in the membranes. Also, the exposed hydrophobic residues can now interact with their targets as well.

Refences

Ames JB, Ishima R, Tanaka T, Gordon JI, Stryer L, Ikura M. Molecular mechanics of calcium-myristoyl switches. Nature 389, 198-202 (1997).

Hurley JB, Spencer M, Niemi GA. Rhodopsin phosphorylation and its role in photoreceptor function. Vision Research 38, 1341-1352 (1998).

Tanaka T, Ames JB, Harvey TS, Stryer L, Ikura M. Sequestration of the membrane-targeting myristoyl group of recoverin in the calcium-free state. Nature 376, 444-447 (1995).