Allurin: Exploring the Activity of a Frog Sperm Chemoattractant in Mammals

Allurin, a 21-kDa protein secreted by the oviduct of female Xenopus frogs, is incorporated into the jelly layers of eggs as they pass single file on their way to the uterus and subsequent spawning. Hydration of the egg jelly layers at spawning releases allurin as a chemoattractant that binds to the midpiece of Xenopus sperm in a dose-dependent manner. Gradients of allurin elicit directed swimming across a porous membrane in two-chamber assays and preferential, up-gradient swimming of sperm in video-microscopic assays. Allurin, purified from X. laevis or produced in recombinant form, also elicits chemotaxis by mouse sperm in two-chamber and video microscopic assays. Allurin binds to mouse sperm at the midpiece and head, a pattern also seen in frog sperm. Western blots suggest the presence of an allurin-like protein in the follicular fluid of mice and humans and peptides that mimic subdomains within allurin elicit chemoattractive behavior in both mouse and human sperm. By sequence homology, allurin is a truncated member of the Cysteine-RIch Secretory Protein (CRISP) family whose members include Crisps 1, 2, and 4, which have been demonstrated to modulate mammalian sperm functions including capacitation, ion channel activity, and sperm–egg binding. Interestingly, allurin contains only two of the three domains found in these full-length CRISP proteins and in this respect is similar to the sperm self-recognition proteins HrUrabin and CiUrabin important in ascidian gamete interactions. These findings suggest that both full-length and truncated CRISP proteins play important reproductive roles in species widely separated in evolutionary time.