Subunit structure of the connexons in hepatocyte gap junctions

Area of Science:

• Cell Biology
• Structural Biology
• Biophysics

Background:

• Gap junctions facilitate intercellular communication in mammalian tissues.
• Connexons, the protein complexes forming gap junctions, are known to have a subunit structure.
• Previous studies suggested hepatocyte connexons are composed of six subunits.

Purpose of the Study:

• To investigate the native structure of connexons in mouse hepatocytes.
• To clarify the observed variations in connexon subunit number and symmetry.
• To determine if apparent structural deviations are artifacts or true biological variations.

Main Methods:

• Cryo-electron microscopy of quick-frozen, freeze-fractured mouse hepatocytes.
• Rotary shadowing and Markham's rotation techniques for high-resolution imaging.
• Analysis of connexon symmetry using tilting experiments and physical models.

Main Results:

• Connexons in native, quick-frozen hepatocyte gap junctions consistently exhibit a subunit structure.
• Observed variations in subunit number (less than six) are attributed to shadowing angles and fracturing artifacts.
• Apparent deviations from hexameric symmetry (approaching tetrameric) are explained by the inclination of the fractured surface relative to the viewing plane.

Conclusions:

• The native structure of connexons in mouse hepatocytes is hexameric.
• Technique-dependent artifacts can lead to misinterpretations of connexon symmetry and subunit count.
• This study refines our understanding of gap junction protein structure and visualization methods.