• On the other
hand, CD45 (also known as leukocyte common
antigen), is a transmembrane protein tyrosine
phosphatase and can remove the inhibitory phosphate from the carboxy terminal tyrosine residue of Src kinases, allowing them to be activated.
• Thus, the balance
between Csk and CD45 determines
the signaling activity of Src kinases following antigen stimulation of the cell; CD45 is required for signal transduction to be effective.
• Src kinases
levels are regulated by ubiquitin attachment
and subsequent degradation in a proteasome.
Cbl, a regulatory E3 ubiquitin ligase, promotes this modification to set max. levels.
•Similar to Csk, Cbl must be phosphorylated to become active; it might also be a source of down-regulation following stimulation by facilitating uquitination of TcR components, other B and T factors.