Conformational studies of G-actin containing bound lanthanide

Area of Science:

• Biochemistry
• Protein Structure
• Biophysical Chemistry

Background:

• G-actin binds adenosine triphosphate (ATP) and divalent cations like calcium (Ca2+).
• Lanthanide (Ln) ions can substitute for Ca2+ at actin's high-affinity cation site.
• Previous studies suggest Ln ions mobilize the ATP nucleotide bound to actin.

Purpose of the Study:

• To investigate the structural and electronic effects of Ln ion binding on G-actin.
• To determine if Ln ion binding induces significant structural changes in G-actin.
• To elucidate the relationship between Ln ion-induced nucleotide mobilization and G-actin structure.

Main Methods:

• Ultraviolet (UV) absorbance spectroscopy to detect changes in G-actin.
• Proteolytic digestion assays to compare actin structural integrity.
• Terbium (Tb3+) luminescence enhancement studies to probe cation site environment.

Main Results:

• Ln ion binding to G-actin causes changes in UV absorbance, attributed to ATP nucleotide mobilization.
• Structural changes in G-actin upon Ln ion substitution for Ca2+ are minimal, not perturbing aromatic chromophores.
• Proteolytic digestion rates are similar for actin with and without bound Ln ions, supporting minor structural alterations.
• UV difference spectroscopy reveals partial exposure of at least one tryptophan (Trp) residue upon nucleotide mobilization.
• Tb3+ luminescence indicates no Trp, tyrosine (Tyr), or phenylalanine (Phe) residues near the high-affinity cation site.

Conclusions:

• Ln ion binding to G-actin primarily mobilizes the bound nucleotide without causing major protein structural changes.
• The observed UV absorbance changes are directly linked to nucleotide mobility, not significant G-actin conformational shifts.
• An exposed tryptophan residue, potentially near the nucleotide binding site, is identified through spectroscopic analysis.