Phospholipase C pathway
Phospholipase C (PLC) is a class of membrane-associated enzymes that cleave phospholipids just before the phosphate group (see figure). It is most commonly taken to be synonymous with the human forms of this enzyme, which play an important role in eukaryotic cell physiology, in particular signal transduction pathways. There are thirteen kinds of mammalian phospholipase C that are classified into six isotypes (β, γ, δ, ε, ζ, η) according to structure. Each PLC has unique and overlapping controls over expression and subcellular distribution. Activators of each PLC vary, but typically include heterotrimeric G protein subunits, protein tyrosine kinases, small G proteins, Ca2+, and phospholipids.
The extensive number of functions exerted by the PLC reaction requires that it be strictly regulated and able to respond to multiple extra- and intracellular inputs with appropriate kinetics. This need has guided the evolution of six isotypes of PLC in animals, each with a distinct mode of regulation. The pre-mRNA of PLC can also be subject to differential splicing such that a mammal may have up to 30 PLC enzymes.
Most of the bacterial variants of phospholipase C are characterized into one of four groups of structurally related proteins. The toxic phospholipases C are capable of interacting with eukaryotic cell membranes and hydrolyzing phosphatidylcholine and sphingomyelin, ultimately leading to cell lysis.
In mammals, PLCs share a conserved core structure and differ in other domains specific for each family. The core enzyme includes a split triosephosphate isomerase (TIM) barrel, pleckstrin homology (PH) domain, four tandem EF hand domains, and a C2 domain. The TIM barrel contains the active site, all catalytic residues, and a Ca2+ binding site. It has an autoinhibitory insert that interrupts its activity called an X-Y linker. The X-Y linker has been shown to occlude the active site, and with its removal PLC is activated.
The genes encoding alpha-toxin (Clostridium perfringens), Bacillus cereus PLC (BC-PLC), and PLCs from Clostridium bifermentans and Listeria monocytogenes have been isolated and nucleotides sequenced. There is significant homology of the sequences, approximately 250 residues, from the N-terminus. Alpha-toxin has an additional 120 residues in the C-terminus. The C-terminus of the alpha-toxin has been reported as a “C2-like” domain, referencing the C2 domain found in eukaryotes that are involved in signal transduction and present in mammalian phosphoinositide phospholipase C.
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