@oristarA,
It works in the larger description in the article. It's relating someone standing tall, reaching one's harms high overhead then bending at the waist and touching one's toes with one's fingertips. In other words, it's saying that one end of the viral protein attaches to a host cell, goes through a transformation, and loops around to attach the other end as well. Here's another description that calls it a "hairpin".
Quote:The first step in fusion involves the high-affinity attachment of the CD4 binding domains of gp120 to CD4. Once gp120 is bound with the CD4 protein, the envelope complex undergoes a structural change, exposing the chemokine binding domains of gp120 and allowing them to interact with the target chemokine receptor.[76][77]
This allows for a more stable two-pronged attachment, which allows the N-terminal fusion peptide gp41 to penetrate the cell membrane.[76][77] Repeat sequences in gp41, HR1 and HR2 then interact, causing the collapse of the extracellular portion of gp41 into a hairpin. This loop structure brings the virus and cell membranes close together, allowing fusion of the membranes and subsequent entry of the viral capsid.[76][77]
wiki
The results of the research indicates that much of the anti-HIV antibody production is ineffective because it produces non-neutralizing antibodies (antibodies that don't destroy the virus) after the host cells are already infected. They've determined that antibodies produced before the gp41 protein completes it's looping, or hairpinning (or toe touching) are effective in neutralizing the virus before it enters the cell and begins replicating.
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