Hyalin

Hyalin is a protein released from the cortical granules of a fertilized animal egg. The released hyalin modifies the extracellular matrix of the fertilized egg to block other sperm from binding to the egg, and is known as the slow-block to polyspermy. All animals have this slow-block mechanism.

Hyalin is a large, acidic protein which aids in embryonic development. The protein has strong adhesive properties which can help with cell differentiation and as a polyspermy prevention component. It forms the hyaline layer which covers the surface of the egg after insemination.

Its physical structure has a major and minor component. One is filamentous, having flexible molecules containing a globular domain head at the end. Its conformation is retained mainly by disulfide bonds, as virtually all cysteine amino acids are found in the disulfide form, but also hydrophobic forces and salt linkages stabilize the molecule. The filament length is about 75 nm long, and the head being club-shaped with a diameter of 12 nm. An isoform of the molecule exists, having a longer filament of 125 nm instead. Both forms of these filaments often fold on themselves, making the protein heterogeneous, resulting in poorly resolved stains on a gel. This makes the exact mass uncertain, as the protein is very difficult to purify. Estimates place the mass at about 350 kDa. About 2-3% of its mass is carbohydrates. Aggregates of hyalin also form by associating the heads of the protein, and hyalin remains accociated with a high, molecular weight core protein throughout purification.

Hyalin mRNA is about 12kb in length. It encodes for approximately 25% acidic residues with only 3.5% basic residues. Within its sequence is a region containing tandem repeats of about 84 amino acids. This sequence is highly conserved between species, and is believed to be the adhesive substrate of hyalin. A recombinant part of this sequence was created and its adhesive properties were tested. It was found to be about as adhesive as native hyalin. Antibodies bound to the recombinant hyalin and blocked its adhesion similar to normal hyalin. The tandem repeat region was then found to be on the filamentous part of hyalin when the antibodies bound to it. As many as 21 of these long repeats can be present, accounting for 230 kDa of the total mass and two-thirds of the filamentous region. These repeats shows no resemblance to anything within the genbank, making hyalin a unique protein.

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