How to: Autophosphorylation

REFERENCES:

Malecka & Peterson. “Face-to-Face, Pak-to-Pak.” (2011) Structure 19(12), pgs 1723 – 1724.

Wang et al. “Structural Insights into the Autoactivation Mechanism of p21-Activated Protein Kinase.” (2011) Structure 19(12), pgs 1752 – 1761. 

                Many kinases require the phosphorylation of a residue within their active site to help maintain a conformation that is compatible with substrate binding/kinase activity.  While some kinases are able to phosphorylate other kinases, many times it is the kinase itself which performs autophosphorylation.  However, it is an interesting question: if the kinase requires phosphorylation to function but it itself must provide the phosphorylation, how does that work?  It is nearly a “chicken or the egg” problem.

                In the issue of Structure published on December 6^th, two papers discuss the de novo phosphorylation of the Pak1 (p21-activated kinase 1) protein.  A commentary offered by Malecka and Peterson discuss the history of common mutations often used by bench scientists to achieve “kinase-dead” proteins that are useful for crystallization as well as an overview of other kinases whose crystal structures reveal dimeric structures where the active loop of one subunit is placed within the active site of the other subunit.  The authors discuss two ways in which kinases can transiently adopt active conformations within the dimer to achieve autophosphorylation in trans: symmetric and asymmetric.  Symmetric structures, such as those seen for the kinases Chk2 and Ire1, place each others activation loops in their active sites while asymmetric structures, such as DAPK3 and IGF1R, have only one subunit place its activation loop within the active site of the other subunit.  

                Pak1, as reported by Wang et al, adopts a symmetric trans-autophosphorylation structure.  Interestingly, the authors also report that the common lysine mutations made within the active site are not 100% kinase-dead so caution should be used when trusting them as such.

Double Duty Inhiitor?

REFERENCE: Hansen et al. Structure (2011) 19, pgs 919 – 929.

Malaria, a disease that causes 1 million deaths per year, is caused by a Plasmodium parasite.  Cysteine proteases (CPs) expressed by the parasite are implicated in key process of both parasitic life stages: liver and blood.  Interestingly, host cell CPs are also integral to infection.  Given the destructive nature of proteases, CPs of both host cells and parasites must be regulated site-specifically and temporarily.  In the July issue of Structure, Hilgenfeld and colleagues discuss the structure of the Plasmodium cysteine protease falcipain-2 (FP-2) in complex with the C terminus of their identified CP inhibitor from Plasmodium berghei (PblCP-C).  PblCP-C has an Ig-like ß sandwich fold and its closest structural relative is identified as chagasin, an I42 inhibitor family member.  Loops L0, L2, L4, and L6 protrude from PblCP-C (shown) into the active site of FP-2, thus occluding substrate binding.  The authors compare the PblCP-C:FP-2 structure to other solved inhibitor complexes and conclude that the major interactions responsible for inhibition are conserved between the structurally unrelated inhibitors, but the PblCP-C L0 interactions with FP-2 are unique to this complex.  Intriguingly, the structure of L0 also explains why PblCP-C is a potent inhibitor of the papain-like protease cathepsin L but not cathepsin B.  Because PblCP is necessary for host cell invasion, it is postulated that this CP inhibitor could block potentially deleterious protease activity at crucial moments, such as host-cell invasion, or inhibit host cell CPs (such as the cathepsin-like caspases).  It also provides a framework for developing small molecule inhibitors of the critically important FP-2.