The research team, led by U. Benjamin Kaupp, from the US-based Marine Biological Laboratory, identified the molecule which allows sodium ions to flow into the sperm cell and, in exchange, transports protons out of the cell.
Previous studies had discovered that eggs from marine invertebrates release a chemical factor that attracts sperm, a process which is called chemotaxis.
The sperm swims up a chemical gradient to reach the egg, assisted by a pulsatile rise in calcium ion (Ca2+) concentration in the sperm tail that controls its beating.
However, a prerequisite for these Ca2+ ions from the sperm's environment to enter the tail is that the sperm cell's pH becomes more alkaline.
The molecule that brings about this change in pH has remained elusive.
In the new study, published in the journal Nature Communications, Kaupp found that the the so-called sodium and proton exchangers is a chimaera that shares structural features with ion channels called pacemaker channels, which control our heartbeat and electrical activity in the brain.
This sodium and proton exchange in the sperm cell, like in the pacemaker channels, is activated by a stretch of positively charged amino acids called the voltage sensor. When the sperm captures chemoattractant molecules, the voltage becomes more negative, because potassium channels open and potassium ions leave the cell.
The voltage-sensor registers this voltage change and the exchanger begins exporting protons from the cell. The cell's interior becomes more alkaline. When this mechanism is disabled, the Ca2+ pulses in the sperm tail are suppressed, and sperms are lost on their voyage to the egg, the researchers said.
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