CKIIβ변이주의 구조적 특성 및 CKII 전효소 내 βsubunit 교환에 관한 연구
Structural analysis of CKIIβ mutants and subunit exchange of CKIIβ within CKII holoenzyme
전효소 CKIIβ 변이주;
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Protein kinase CKII is composed of two catalytic (α or α′) subunits and two regulatory (β) subunits. The CKIIβ subunit is thought to mediate tetramer formation, to interact with other target proteins, and to modulate the catalytic activity of CKIIα subunit. In the previous study, five CKIIβ mutants defective for L41 binding were isolated by using the reverse two-hybrid system. In the two-hybrid test, these mutants still had binding affinity to CKIIα, but three point mutants R26, R75 and R83 did not associate with wild-type CKIIβ. In addition, maltose-binding protein (MBP) pull-down assay showed that R83 associated with CKIIα and CKIIβ, and that R26 and R75 associated with CKIIα but not with the wild-type CKIIβ, indicating that CKIIβ homodimerization is not a prerequisite for its binding to CKIIα or target proteins. In the present study, structural analysis with known CKII crystal structures suggests that Lys-147 and Met-195 may be important for β-β dimerization. Mutation of hydrophilic Lys-147 to hydrophobic Met in R26, which does not stimulate the catalytic activity of CKIIα, may affect the conformation of zinc finger motif. Mutation of Lys-139 in the zinc finger motif to Arg in R75 would not change the conformation of the motif. Instead, this Arg would introduce internal hydrogen bonds around the motif, stabilizing its conformation and then stimulating the CKIIα activity more effectively than wild-type CKIIβ. Addition of R26 to CKII holoenzyme inhibited the catalytic activity of CKII, suggesting that R26 can substitute wild-type CKIIβ in CKII holoenzyme. Pull-down experiment using MBP-R26 confirmed the fact that β subunit in CKII holoenzyme could be exchanged with R26.