Differences between cytoplasmic surfaces of deep-etched heart and liver gap junctions

Area of Science:

• Cell Biology
• Biochemistry
• Structural Biology

Background:

• Gap junctions mediate intercellular communication in tissues.
• Understanding the structural basis of gap junction diversity is crucial for tissue-specific function.
• Previous studies suggested potential differences in gap junction proteins between tissues.

Purpose of the Study:

• To compare the ultrastructure of cytoplasmic surfaces of isolated rat heart and liver gap junctions.
• To investigate the impact of proteolysis on gap junction structure.
• To determine if structural differences indicate distinct protein components.

Main Methods:

• Rapid freezing, freeze fracture, deep etching, and double-axis rotary replication of isolated gap junction pellets.
• Isolation of cardiac gap junctions with and without phenylmethylsulfonylfluoride (PMSF) to control for proteolysis.
• Analysis of protein subunit molecular weights (Mr).

Main Results:

• Unproteolyzed cardiac gap junctions (Mr 44,000–47,000) exhibited particulate cytoplasmic surfaces (CS).
• Proteolyzed cardiac junctions (Mr 29,500) and liver junctions (Mr 28,000) showed nonparticulate CS.
• Cardiac junctions, unlike liver junctions, possess a urea-resistant fuzzy CS coating.

Conclusions:

• The cytoplasmic surfaces of heart and liver gap junctions display significant structural differences.
• These structural variations strongly suggest that the protein components of cardiac and hepatic gap junctions are distinct.
• Proteolysis during isolation significantly alters the observed ultrastructure of gap junction cytoplasmic surfaces.