Co-Conserved Features Associated with cis Regulation of ErbB Tyrosine Kinases
I'll give a quick summary of it here. (Don't worry, this isn't a new direction for this blog.)
This is a study of the structural mechanisms of a certain protein family, called ErbB or EGFR (epidermal growth factor receptor), which is frequently involved in cancer. This family belongs to a protein superfamily called protein kinases.
Biochemistry backgroundKinases are enzymes which perform a type of post-translational modification, phosphorylation: The kinase transfers a phosphate group from adenosine triphosphate (ATP) to another substrate molecule, leaving adenosine diphosphate (ADP) and the phosphorylated substrate.
Protein kinases are kinases that act on protein substrates, i.e. the phosphorylated molecule is another protein. The substrate could even be another protein kinase, so activation of the first protein kinase causes it to phosphorylate and activate another protein kinase, and so on. This is a type of signal transduction.
Signal transduction is how the cell senses and reacts to its environment, and also its own internal conditions. In the case of ErbB and other receptor tyrosine kinases, the signal starts at the surface of the cell (e.g. epidermal growth factor binds to the extracellular portion of EGFR) and activates the kinase, which then begins sending these phosphorylation signals. These signals are then relayed throughout the cell to trigger other activities, such as cell division or the transcription of certain genes.
What happens if a protein kinase gets "locked" into the active state, somehow? In the case of EGFR, it's as if the cell thinks it's constantly receiving the growth factor. If this signal isn't blocked by another "gatekeeper" in the cell, then the cell will grow uncontrollably -- and become cancer.
How the enzyme worksProtein kinases (PKs) all consist of two large lobes connected by a flexible hinge. Between the lobes is a binding pocket for ATP; this molecule binds inside the smaller lobe (N-terminal lobe, or N-lobe). The larger lobe (C-terminal lobe or C-lobe) provides a binding site for another protein, which will be the kinase's substrate.
The general mechanism of all protein kinases goes like this:
- The kinase is initially in an inactive state, with the hinge "open" and the two lobes a bit further apart. Since ATP binds in the N-lobe and the substrate binds to the C-lobe, no phosphate is transferred when the two lobes are apart like this.
- By some mechanism (it varies between different kinase families), the two lobes are brought closer together, and the kinase becomes active.
- ATP binds to the ATP binding pocket, a substrate binds to the C-lobe, some amino acids shift, and a phosphate group is detached from ATP and reattached to a specific amino acid on the substrate.
- The ADP and phosphorylated substrate are released.
How we think ErbB kinases workIn the ErbB family, it's not just the two lobes of the kinase domain that are involved in activating the enzyme -- the adjacent sections of the protein, outside the kinase domain, are also involved.
The long C-terminal tail wraps back around the entire kinase domain and associates with the N-lobe, tethered in place by a few residues in the N-lobe and the other N-terminal flanking region (the juxtamembrane segment, between the kinase domain and the cell membrane). The C-tail is placed so that it can influence the movement and relative positioning of the N- and C-lobes, and therefore regulate the activation of the kinase.
We also examined the locations of two EGFR mutations (S768I and L861Q) that have been previously identified as occurring frequently in cancers, mapping them onto the structure. These mutations appear in locations that would disrupt the switching mechanism we proposed -- breaking necessary interactions, or forming new interactions that shouldn't be there for proper EGFR function.
If you'd like to know more, read about it here.