Caspase-3 Regulatory Mechanisms: Difference between revisions

Caspases are '''''c'''''ysteine-'''''asp'''''artic acid prote'''''ases''''' and are key protein facilitators for the faithful execution of apoptosis or programmed cell death. Dysregulation in the apoptotic pathway has been implicated in a variety of diseases such as neurodegeneration, cancer, heart disease and some metabolic disorders.  Because of the crucial role of caspases in the the apoptotic pathway, abnormalities  in their functions would cause haywire in the apoptotic cascade and can be deleterious to the cell.  Caspases are thus being considered as therapeutic targets in apoptosis-related diseases.

Any apoptotic signal received by the cell causes the activation of initiator caspases (-8 and -9) by associating with other protein platforms to form a functional holoenzyme.  These initiator caspases then cleave the executioner caspases -3, -6, and -7.  Caspase-3 specifically functions to cleave downstream apoptotic targets as well as both caspase-6 and -7, which in turn cleave their respective targets to induce cell death.  Aside from being able to activate caspase-6 and -7, caspase-3 also regulates caspase-9 activity, operating via a feedback loop.  This dual action of caspase-3 confers its distinct regulatory mechanisms, resulting in a wider extent of its effects in the apoptotic cascade.

== Overview of Caspase-3 Structure ==

===Dimer Formation ===

Caspase-3 shares many structural characteristics with other caspases.  It is synthesized in the cell in its zymogen form, consisting of an N-terminal prodomain followed by a large and small subunit linked to each other by an intersubunit linker. Like other executioner caspases, caspase-3 has a short N-terminal prodomain the function of which remains unknown.  Maturation of the enzyme involves at least two cleavage events- one to remove the N-terminal prodomain and the other to cleave the intersubunit linker.  These two cleavage events have been shown to occur in a sequential fashion, with the cleavage between the small (p12) and large (p17) subunits preceding the pro domain removal.  The high specificity of caspases directs the cleavage of the intersubunit linker at specific aspartate residues and generates the mature form of the enzyme. Caspase-3 in its functional form is a <scene name='Sandox_Bay_Serrano/Scene01_dimer/3'>heterotetramer</scene>; each heterodimer is formed and stabilized by hydrophobic interactions between the large and small subunit.  ß-sheets from each heterodimer then interact resulting in a 12-stranded <scene name='Sandox_Bay_Serrano/Scene01_dimer/2'>ß-sheet structure</scene>, around which α-helices are positioned.   

Since all caspases have very similar active site chemistry, other regions in the protein that can confer either activation or inhibition to the enzyme needs to be explored.  For example, exosites in caspase-7 have been identified and were observed to improve activity (Boucher, Blais et al. 2012). Caspase-7 also has an inhibitory allosteric site that could bind with the small molecule FICA, resulting in a zymogen-like conformation (Hardy, Lam et al. 2004) and abolishing activity.

Although there are still no evident exosites found in caspase-3, some allosteric sites, most of which are located on the dimer interface, have been interrogated by mutagenesis and have been shown to modulate the activity of caspase-3 or even procaspase-3. Although only procaspase-3 was detected with little activity because the orientation of ILA (prematured L2 loop) and ILB loop cannot form an active site pocket, it is sill quite interesting to discover how these mutations are able to rescue activity of the zymogen form (Bose, Pop et al. 2003).

It was found recently that changing other critical residues (V266H, Y197C, E124A) on the dimer interface might play an important role on inhibition of caspase-3 by altering hydrogen bond patterns or through a relay of subtle conformational changes that spreads across the whole dimer.

Caspase-3 has been reported to be phosphorylated at Ser-150 by p38-MAPK and directly inhibits its activity.  Conversely, phosphorylation of caspase-3 by protein kinase C zeta (PKC-ζ) was shown to promote autocleavage and activation.

=== Natural Inhibitors===

<scene name='Caspase-3_Regulatory_Mechanisms/Bir2/1'>Casp3 bound to BIR2 domain</scene>

X-linked inhibitor of apoptosis proteins (XIAP) contains the second baculovirus IAP repeat domain (BIR2) targeting caspase-3 and caspase-7. The <scene name='Caspase-3_Regulatory_Mechanisms/Bir2/2'>BIR2</scene> domain sits directly in the active site of caspase-3 and completely inhibits the protein. The region binding the active site runs in the opposite direction of normal caspase-3 substrates, thus occupying P1 through P4 but avoiding cleavage by the protease. This unique method of inhibition is a critical regulatory mechanism used in cells to control apoptotic caspase activity.

=References=