 Solution

In lane 3, detergent removes the lipid bilayer barrier while both partner preparations are present. Their binding regions can then encounter one another even if they originally faced inward or were on poorly apposed lipid bilayers. The strong remaining full-length signal is consistent with extensive formation of protected SNARE complexes.

Thus detergent does not prevent partner association under the displayed conditions; it removes the topological restriction on that association. This lane is the normalization reference, not a direct measurement that 100% of all initially supplied protein is active. The weak signal in the detergent-treated partner-free control also argues against detergent simply preventing all cleavage.