Protein Kinase C

-Modeling the Active Site

Protein kinase C transduces the cellular signals that promote lipid hydrolysis. This 80kDa enzyme is recruited to the plasma membrane by diacylglycerol and, in many cases, by calcium. The enzyme is activated by diacylglycerol and phospholipid (usually PS) and is thought to undergo a conformational change upon binding to the membrane. PKC phosphorylates a variety of target proteins which control growth and cellular differentiation.

The structure of PKC is not known, but the isozymes of PKC are homologous with cAMP-dependent protein kinase (protein kinase A), and Orr and Newton have modeled the catalytic domain of the PKC beta-II isozyme, based on the structure of PKA (J. Biol. Chem. 269, 8383 (1994)).

We'll begin our examination of this enzyme by displaying the protein as a ribbon structure .

The Orr/Newton model includes a bound "pseudosubstrate" peptide , derived from the N-terminal pseudosubstrate domain of PKC, as well as a molecule of ATP , bound deep in the active cleft and covered by the bound pseudosubstrate peptide. Two Mn ions . are coordinated by the terminal phosphates of the ATP.

The modeled structure of the catalytic domain of PKC includes several large alpha helices , a twisted beta sheet , and a number of smaller alpha-helical segments .

PKC is capable of autophosphorylation, and Thr-500 is phosphorylated in this model .

The location of Trp-493 in this model is similar to that of Trp-196 in PKA. This latter residue has been shown to be part of the interface between the catalytic and regulatory subunits of PKA, and the same may be true for Trp-493 in PKC.

Let's take a closer look at the bound pseudosubstrate peptide The following button rotates the image by 120 degrees: , and this button will allow us to zoom in and examine the bound peptide more closely .

The pseudosubstrate peptide possesses all the features of a target peptide, but contains an alanine (Ala-25) instead of a Ser or Thr at the "phospho-acceptor site " (shown in red-orange). The phospho- acceptor site is bordered by basic residues, including a Lys residue (orange) and several Arg residues (yellow).

PKC target peptides typically possess a hydrophobic residue C-terminal to the phosphoacceptor site, and in the case of the pseudosubstrate peptide this residue is a leucine (violet).

You may wish to manipulate this image yourself: