Crystal structure of a nuclear actin ternary complex.

Area of Science:

• Cell Biology
• Structural Biology
• Biochemistry

Background:

• Actin polymerizes into filaments (F-actin) in the cytoplasm.
• Nuclear actin (N-actin) is involved in nucleic acid transactions and chromatin remodeling.
• The structure and polymerization regulation of N-actin in the nucleus are unknown.

Purpose of the Study:

• To determine the crystal structure of nuclear actin (N-actin).
• To elucidate the mechanisms regulating N-actin polymerization and function within the nucleus.

Main Methods:

• X-ray crystallography was used to determine the structure of the N-actin complex.
• Structural analysis focused on interactions with actin-related protein 4 (Arp4) and the Swr1 chromatin remodeler.

Main Results:

• The crystal structure of N-actin complexed with Arp4 and the HSA domain of Swr1 was solved.
• N-actin's polymerization is inhibited by sequestration of its barbed end and inner face by Arp4 and Swr1.
• N-actin's nucleotide-binding pocket is occluded, preventing ATP binding and regulation.

Conclusions:

• Nuclear actin possesses distinct structural features compared to cytoplasmic actin.
• These structural differences explain N-actin's inability to polymerize and its unique regulatory mechanisms in the nucleus.
• The findings provide a structural basis for N-actin's role in nuclear processes.