Self-nonself Recognition and Lysin System in Fertilization of the Ascidian Halocynthia roretzi

Abstract

Accumulating evidence suggests that a 70-kDa component (HrVC70) of the vitelline coat (VC) of Halocynthia roretzi eggs associates with the VC in the process of oocyte maturation, which is a step in the acquisition of the self-nonself recognition system. A cDNA clone encoding the precursor for HrVC70 was isolated. HrVC70 is made up of 12 EGF-like repeats and is biosynthesized as a 120-kDa-precursor protein that contains 13 EGF-like repeats and a ZP homologous region. TA cloning analysis of HrVC70 revealed that amino acid residues located between the third and fourth Cys residues in each EGF domain and in the connecting region of EGF domains are substituted in each individual. Furthermore, the number of non-self sperm that bound to a VC70-agarose bead was higher than that of self-sperm. These results suggest that HrVC70 is a candidate self-nonself recognition molecule functioning in the fertilization of H. roretzi Several lines of evidence indicate that HrVC70 is degraded by the sperm proteasome in a ubiquitin-dependent manner during fertilization of H. roretzi.