Despite central roles of egg coat proteins in gamete recognition, their functions and composition are poorly understood. Here, we report that the proteome of the egg coat in the solitary ascidian Ciona intestinalis, called vitelline coat (VC) fraction, contains more than 800 proteins identified by mass spectrometry-based analyses. Over 100 proteins were enriched in the VC fraction compared with the VC-free egg proteome. The most abundant component in the VC was an apolipoprotein-like protein. The VC contained multiple homologs of mammalian zona pellucida (ZP) proteins, the number of which was unexpectedly large and most of which possessed epidermal growth factor-like repeats. Furthermore, the present study revealed that two fibrinogen-like proteins, v-Themis-A and -B, both of which are expressed in the VC, are the molecules responsible for the two self-sterility loci that were identified by our previous genetic study in this species
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