second protein kinase C conserved region 1 (C1 domain) found in the classical (or conventional) protein kinase C (cPKC) family
PKCs are classified into three groups (classical, atypical, and novel) depending on their mode of activation and the structural characteristics of their regulatory domain. cPKCs are potent kinases for histones, myelin basic protein, and protamine. They depend on calcium, DAG (1,2-diacylglycerol), and in most cases, phosphatidylserine (PS) for activation. There are four cPKC isoforms, named alpha, betaI, betaII, and gamma. PKC-alpha is expressed in many tissues and is associated with cell proliferation, apoptosis, and cell motility. It plays a role in the signaling of the growth factors PDGF, VEGF, EGF, and FGF. Abnormal levels of PKC-alpha have been detected in many transformed cell lines and several human tumors. In addition, PKC-alpha is required for HER2 dependent breast cancer invasion. The PKC beta isoforms (I and II), generated by alternative splicing of a single gene, are preferentially activated by hyperglycemia-induced DAG (1,2-diacylglycerol) in retinal tissues. This is implicated in diabetic microangiopathy such as ischemia, neovascularization, and abnormal vasodilator function. PKC-beta also plays an important role in VEGF signaling. In addition, glucose regulates proliferation in retinal endothelial cells via PKC-betaI. PKC-beta is also being explored as a therapeutic target in cancer. It contributes to tumor formation and is involved in the tumor host mechanisms of inflammation and angiogenesis. PKC-gamma is mainly expressed in neuronal tissues. It plays a role in protection from ischemia. Members of this family contain two copies of C1 domain. This model corresponds to the second one. The C1 domain is a cysteine-rich zinc binding domain that does not bind DNA nor possess structural similarity to conventional zinc finger domains; it contains two separate Zn(2+)-binding sites.