Structural studies of arthrin: monoubiquitinated actin

Area of Science:

• Muscle biology
• Protein ubiquitination
• Structural biology

Background:

• Actin is the main component of muscle thin filaments.
• Protein ubiquitination is a key post-translational modification regulating protein function.
• The structure and precise location of ubiquitinated actin (arthrin) in muscle filaments were previously unknown.

Purpose of the Study:

• To determine the structure and in situ location of arthrin within insect muscle thin filaments.
• To elucidate the specific site of ubiquitination on actin.
• To provide insights into the potential function of actin ubiquitination.

Main Methods:

• Immunolabeling of insect muscle thin filaments with a ubiquitin antibody.
• Three-dimensional reconstruction of frozen-hydrated arthrin filaments using a novel segmentation algorithm.
• Difference mapping between arthrin and actin filament reconstructions.
• Peptide mapping to identify the ubiquitination site.
• Molecular modeling of arthrin monomers and filaments.

Main Results:

• Ubiquitinated actin (arthrin) was identified in insect muscle thin filaments, with ubiquitination occurring at every seventh subunit along the filament long-pitch helices.
• Three-dimensional reconstruction revealed ubiquitin's location on actin sub-domain 1, opposite the myosin binding site.
• Peptide mapping confirmed ubiquitination at lysine 118 (Lys118) of actin.
• Molecular modeling suggested that the ubiquitin moiety attached to Lys118 adopts a limited number of conformations stabilized by an interface with actin.

Conclusions:

• The study determined the precise structure and location of arthrin in muscle thin filaments.
• Ubiquitination of actin occurs at Lys118 and is spatially distinct from the myosin binding site.
• The findings suggest that actin ubiquitination may play a role in regulating muscle contractile activity.