How many connexin subunits form one complete synaptic channel?

Question

BlackTom AI · Accepted Answer

A gap junction (electrical synapse) channel is formed by two apposed hemichannels (also called connexons), and each hemichannel is a hexamer of connexin proteins. That hexameric stoichiometry is the key structural fact that determines the total number of connexin subunits in a complete, docked channel.

1) Relevant concepts:
- Connexin: the protein subunit that assembles into hemichannels.
- Connexon (hemichannel): an assembly of six connexin subunits (hexamer).
- Gap junction channel (electrical synapse): formed when two hemichannels from adjacent cells dock head-to-head.

2) Step-by-step reasoning:
- Step 1: One connexon contains 6 connexin subunits (by definition; connexons are hexamers).
- Step 2: A functional gap junction channel results from docking of two connexons, one contributed by each of the two neighboring cells.
- Step 3: Therefore, total connexin subunits = 6 (from cell A) + 6 (from cell B) = 12.

3) Calculation (explicit):
- 6 subunits/hemichannel × 2 hemichannels = 12 subunits.

4) Why this is correct:
Structural studies (electron microscopy and biochemical crosslinking) and functional analyses consistently show connexons are hexameric. Docking two hexameric connexons yields a dodecameric channel (12 connexins) spanning both membranes and the extracellular gap, allowing direct cytoplasmic continuity.

5) Why other counts would be incorrect:
- 6 would describe a single undocked hemichannel, not a complete intercellular channel.
- Any other numbers (e.g., 8, 10, 24) are inconsistent with the established hexamer + hexamer arrangement of vertebrate connexin-based gap junctions; they do not match the known molecular assembly.

Conclusion: Because two hexameric connexons dock to form a gap junction channel, there are 6 + 6 = 12 connexin subunits in one complete synaptic (gap junction) channel. This matches the provided answer: 12.

How many connexin subunits form one complete synaptic channel?单项选择题

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