The major structural proteins of cod (Gadus morhua) eggshells and protein crosslinking during teleost egg hardening

The highly hydrophobic protein aggregate which constitutes the fish eggshell has for the first time been quantitatively solubilized. This study shows that the nonactivated eggshell from cod is composed primarily of only three protein monomers, designated alpha (74 kDa) beta (54 kDa) and gamma (47 kDa). Protein extraction studies of the eggshells before and after egg activation demonstrate that egg hardening is accompanied by a 10-fold decline in total protein solubility, which is due to nonextraction of the alpha, beta, and gamma chains. When present during the egg activation process monodansylcadaverine (MDC-a fluorescent lysine analog) inhibits eggshell hardening and at the same time becomes covalently incorporated into the eggshell. This MDC incorporation is calcium-dependent and suggests the induction of a perivitelline transglutaminase activity after egg activation. (Transglutaminases catalyze the formation of an amide bond (isopeptide bond) between the gamma-carbonyl group of glutamine and the epsilon-amino group of lysine with release of ammonia. Crosslinks between proteins are generated when the two amino acid residues are located on different proteins.) Protein solubilization studies and NaDodSO4 gel analysis of the eggshell proteins from eggs subjected to 5 mM MDC during egg activation, reveal that when eggshell hardening is blocked by MDC, the three main eggshell proteins remain extractable even after egg activation. Simultaneously we observed a covalent incorporation of MDC into the gamma protein.