Bax inhibitor 1 increases cell adhesion through actin polymerization: Involvement of calcium and actin binding

Bax inhibitor 1 (BI-1), a transmembrane protein with Ca²⁺ channel-like activity, has antiapoptotic and anticancer activities. Cells overexpressing BI-1 demonstrated increased cell adhesion. Using a proteomics tool, we found that BI-1 interacted with γ-actin via leucines 221 and 225 and could control actin polymerization and cell adhesion. Among BI-1 ^-/- cells and cells transfected with BI-1 small interfering RNA (siRNA), levels of actin polymerization and cell adhesion were lower than those among BI-1^+/+ cells and cells transfected with nonspecific siRNA. BI-1 acts as a leaky Ca²⁺ channel, but mutations of the actin binding sites (L221A, L225A, and L221A/L225A) did not change intra-endoplasmic reticulum Ca²⁺, although deleting the C-terminal motif (EKDKKKEKK) did. However, store-operated Ca²⁺ entry (SOCE) is activated in cells expressing BI-1 but not in cells expressing actin binding site mutants, even those with the intact C-terminal motif. Consistently, actin polymerization and cell adhesion were inhibited among all the mutant cells. Compared to BI-1 ^+/+ cells, BI-1^-/- cells inhibited SOCE, actin polymerization, and cell adhesion. Endogenous BI-1 knockdown cells showed a similar pattern. The C-terminal peptide of BI-1 (LMMLILAMNRKDKKKEKK) polymerized actin even after the deletion of four or six charged C-terminal residues. This indicates that the actin binding site containing L221 to D231 of BI-1 is responsible for actin interaction and that the C-terminal motif has only a supporting role. The intact C-terminal peptide also bundled actin and increased cell adhesion. The results of experiments with whole recombinant BI-1 reconstituted in membranes also coincide well with the results obtained with peptides. In summary, BI-1 increased actin polymerization and cell adhesion through Ca²⁺ regulation and actin interaction.