The catalytic domain of inositol‐1,4,5‐trisphosphate 3‐kinase‐a contributes to ITPKA‐induced modulation of F‐actin

Abstract

Inositol-1,4,5-trisphosphate-3-kinase-A (ITPKA) has been considered as an actin bundling protein because its N-terminal actin binding domain (ABD) induces formation of linear actin bundles. Since in many cancer cell lines ITPKA is essential for formation of lamellipodia, which consist of cross-linked actin filaments, here we analyzed if full length-ITPKA may induce formation of more complex actin structures. Indeed, we found that incubation of F-actin with ITPKA resulted in formation of dense, branched actin networks. Based on our result that ITPKA does not exhibit an additional C-terminal ABD, we exclude that ITPKA cross-links actin filaments by simultaneous F-actin binding with two different ABDs. Instead, stimulated-emission-depletion-microscopy and measurement of InsP3 Kinase activity give evidence that that N-terminal ABD-homodimers of ITPKA bind to F-actin while the monomeric C-termini insert between adjacent actin filaments. Thereby, they prevent formation of thick actin bundles but induce formation of thin branched actin structures. Interestingly, when embedded in this dense actin network, InsP3 Kinase activity is doubled and the product of InsP3 Kinase activity, Ins(1,3,4,5)P4 , inhibits spontaneous actin polymerization which may reflect a local negative feedback regulation of InsP3 Kinase activity. In conclusion, we demonstrate that not only the ABD of ITPKA modulates actin dynamics but reveal that the InsP3 Kinase domain substantially contributes to this process.

Inositol-1,4,5-trisphosphate 3-kinase-A (ITPKA) is frequently over-expressed and functions as an oncogene in several tumor types

“…In cells it is exclusively bound to F-actin resulting in cross-linking of actin filaments [12,23]. Thus, based on this function and on its InsP 3 Kinase activity, ITPKA has two very distinct functions, regulating both, calcium signaling and actin dynamics.…”

“…The bulky C-terminus, which includes the InsP 3 Kinase-domain, acts as spacer between actin filaments resulting in formation of loose networks of F-actin bundles (Fig. 1B; [23]).…”

Inositol 1,4,5-trisphosphate 3-kinase A overexpressed in mouse forebrain modulates synaptic transmission and mGluR-LTD of CA1 pyramidal neurons

“…Inositol 1,4,5-trisphosphate 3-kinase A (IP 3 K-A) has emerged as an important molecule for synaptic plasticity owing to its abilities to convert inositol trisphosphate (IP 3 ) to inositol tetrakisphosphate (IP 4 ) as well as to bind to F-actin and microtubules [ 1 – 6 ]. Although IP 3 K-A is thought to be an intracellular calcium (Ca 2+ ) signaling regulator through the IP 3 receptor (IP 3 R) pathway [ 1 , 2 ], IP 3 K-A deletion in knockout (KO) mice does not change the net content of IP 3 [ 7 ] but results in an increase in IP 3 turnover in hippocampal synaptosomes [ 8 ].…”

Focusing super resolution on the cytoskeleton

“…Others have used STED to examine the actin-like MreB protein in bacteria 91 and the reorganization of actin filament arrays during activation of natural killer cells and T cells 92 , 93 . It has allowed the visualization of myosin mini-filament formation in mammalian non-muscle cells 94 and insight into the regulation of the actin cytoskeleton by intracellular signaling proteins 95 , 96 . Microtubules have been examined using STED in primary cilia 97 , and the interphase microtubule array has been examined in muscle and non-muscle cells 98 , 99 .…”