In Quest of “Oncogenic” Actin Kinases (AKs): CK1 (Casein-Kinase 1) and PAK1

Back to 1982, when we worked at Max-Planck-Institute in Munich, Germany, we found a very peculiar kinase in Physarum which selectively phosphorylates a capping protein called Cap42 (1).  Cap42 is a hetero-dimer consisting of two subunits called Cap42 (a) and Cap42 (b).  Cap42 caps actin-filaments at the “barbed” end to block actin polymerization at this end, leading to a rapid depolymerization at the opposite (“pointed”) end of  filaments. Thus, Cap 42 causes a rapid shortening of actin filaments. This biological activity is very similar to that of Physarum fragmin and mammalian gelsolin, Ca²⁺-dependent capping/severing proteins. 

However, the capping activity of Cap42 per se is Ca²⁺-independent, and when Cap42 is phosphorylated by this peculiar kinase in Physarum, its capping activity becomes Ca²⁺-dependent. The phosphorylation takes place on Cap42 (b) only when it forms the complex with Cap42 (a), and the phosphorylation is inhibited by actin and Ca2+.  This kinase was originally called Cap42-kinase, but later renamed Actin-Fragmin Kinase (AF-kinase), because Cap42 (b) is indistinguishable from actin, while Cap42 (a) is indistinguishable from fragmin, by a few biochemical criteria (2).  AF-kinase is a protein of 80 kDa. 

Interestingly, around 1986, it was found that actin is phosphorylated by a mammalian kinase called CK1 (casein kinase 1) in vitro (3), and far later (in 2003) actin is phosphorylated in mammalian cells when they are treated with Calyculin A, a phosphatase inhibitor and Ca²⁺channel-blocker from a marine organism (4), strongly suggesting that the oncogenic kinase CK1 is a mammalian version of Physarum AK-kinase, although the Physarum AK-kinase per se fails to phosphorylate casein.  Since both CK1 and Calyculin A are oncogenic, it has been speculated that the phosphorylation of actin by these kinases also causes a malignant transformation of cells.  How is the p-actin (phosphorylated actin) oncogenic?  

In this context, it is of great interest to note that the phosphorylation sites of actin are around Thr 201-203 of actin, corresponding to the pointed end of actin filaments, to which DNase binds (2). Furthermore, DNase blocks the actin phosphorylation by these kinases. More interestingly, around 1988, a mutant of beta-actin was found to be "non-polymerizable" and oncogenic (5). In this mutant, Gly 244 is replaced by Asp, and this site is located on the “pointed” end of actin, very close to Thr 201-203 in 3D level, suggesting again that the extra negative charge(s) at the “pointed” end of actin could render actin molecule to be "non-polymerizable" and oncogenic. 

Thus, it would be of great interest to test if the Thr 201-203 Asp mutant of actin causes a malignant transformation of normal cells. It would also be of interest to test if a CK1 inhibitor blocks the Calyculin A-induced phosphorylation of actin in mammalian cells, and their oncogenic growth as well. Furthermore, we are very keen to see whether PAK1 (a myosin kinase) is involved in the activation of CK1 (an actin-kinase) or vice versa. 

If we could confirm that the p-actin is oncogenic, the CK1 inhibitor would be another good candidate for cancer therapy, as are CK2 inhibitors such as CX-4945 that eventually block PAK1 (6).  

Finally, to our hugely pleasant surprise, a Greek group  claimed that opioids-activated PI-3 kinase forms a complex with PAK1 in OK (opossum kidney) cells, and PAK1 phosphorylates actin, probably directly (7), although the phosphorylation site still remains to be clarified. 

References: 
1. Maruta H, Isenberg G, Schreckenbach T, Hallmann R, Risse G, Shibayama T, Hesse J. Ca2+-dependent actin-binding phosphoprotein in Physarum polycephalum. I. Ca2+/actin-dependent inhibition of its phosphorylation. J Biol Chem. 1983; 258(16):10144-50.
2. Gettemans J, De Ville Y, Vandekerckhove J, Waelkens E. Physarum actin is phosphorylated as the actin-fragmin complex at residues Thr203 and Thr202 by a specific 80 kDa kinase. EMBO J. 1992 ; 11(9):3185-91.
3. Shibayama T, Shinkawa K, Nakajo S, Nakaya K, Nakamura Y. Phosphorylation of muscle and non-muscle actins by casein kinase 1 in vitro. Biochem Int. 1986; 13(2):367-73.
4. Gu L, Zhang H, Chen Q, Chen J. Calyculin A-induced actin phosphorylation and depolymerization in renal epithelial cells. Cell Motil Cytoskeleton. 2003; 54(4):286-95.
5. Taniguchi S¹, Sagara J, Kakunaga T. Deficient polymerization in vitro of a point-mutated beta-actin expressed in a transformed human fibroblast cell line. J Biochem. 1988. 103(4):707-13.
6. Kim YB, Shin YJ, Roy A, Kim JH.　The Role of the Pleckstrin Homology Domain-Containing Protein CKIP-1 in Activation of p21-activated Kinase 1 (PAK1). J Biol Chem. 2015 Jul 9.

7. Papakonstanti EA, Stournaras C. Association of PI-3 kinase with PAK1 leads to actin phosphorylation and cytoskeletal reorganization. Mol Biol Cell. 2002; 13(8):2946-62.