A specific domain of Gialpha required for the transactivation of Gialpha by tubulin is implicated in the organization of cellular microtubules.
G(s)alpha, G(i)alpha(1), and G(q)alpha subunits bind tubulin with high affinity, whereas transducin (G(t)alpha) does not. The interaction between tubulin and Galpha, which also involves the direct transfer of GTP from tubulin to Galpha (transactivation), is not yet fully understood. This study, using chimeras of G(i)alpha and G(t)alpha, showed that the G(i)alpha (215-295) segment converted G(t)alpha to bind to tubulin and this chimera (chimera 1) could be transactivated by tubulin. Insertion of G(t)alpha (237-270) into chimera 1 to form chimera 2 resulted in a protein that, like G(t)alpha, did not bind tubulin. Thus, it was thought that the G(i)alpha (237-270) domain was essential to modulate the binding of G(i)alpha(1) to tubulin. Surprisingly, when domain (237-270) of G(i)alpha was replaced by G(t)alpha (237-270) to form chimera 3, the chimera bound to tubulin with a similar affinity (K(D) congruent with 120 nm) as wild-type G(i)alpha(1). However, even though chimera 3 displayed normal GTP binding, it was not transactivated by GTP-tubulin. Furthermore, when these chimeras were expressed in COS-1 cells, cellular processes in cells overexpressing G(i)alpha(1) or chimera 1 were more abundant and longer than those in native cells. Galpha was seen throughout the length of the process. Morphology of cells expressing chimera 2 was identical to controls. Consistent with the role of Chimera 3 as a "dominant negative" Galpha, cells transfected with chimera 3 had only few truncated processes. This study demonstrates that although G(i)alpha (237-270) is not obligatory for the binding of G(i)alpha to tubulin, it is crucial for the transactivation of Galpha by tubulin. These results also suggest that the transactivation of Galpha by tubulin may play an important role in modulating microtubule organization and cell morphology.
Chen
Ning-Fang
NF
Departments of Physiology & Biophysics and Psychiatry, University of Illinois at Chicago, Chicago, Illinois 60612, USA.
Yu
Jiang-Zhou
JZ
Skiba
Nikolai P
NP
Hamm
Heidi E
HE
Rasenick
Mark M
MM
D000818:Animals; D001665:Binding Sites; D019556:COS Cells; D048429:Cell Size; D019204:GTP-Binding Proteins / Q000096:biosynthesis* / Q000737:chemistry / Q000235:genetics; D006801:Humans; D008870:Microtubules / Q000378:metabolism* / Q000648:ultrastructure; D011485:Protein Binding; D017434:Protein Structure, Tertiary / Q000235:genetics; D021122:Protein Subunits / Q000235:genetics; D011993:Recombinant Fusion Proteins; D015533:Transcriptional Activation; D015293:Transducin / Q000235:genetics / Q000378:metabolism; D014162:Transfection; D014404:Tubulin / Q000502:physiology*
D021122:Protein Subunits; D011993:Recombinant Fusion Proteins; D014404:Tubulin; D019204:GTP-Binding Proteins; D015293:Transducin