During fertilization, the sea urchin sperm acrosome reaction (AR), an
ion channel-regulated event, is triggered by
glycoproteins in egg jelly (EJ). A 210-kD sperm
membrane glycoprotein is the receptor for EJ (REJ). This conclusion is based on the following data: purified REJ binds species specifically to EJ dotted onto
nitrocellulose, an mAb to REJ induces the sperm AR, antibody induction is blocked by purified REJ, and purified REJ absorbs the AR-inducing activity of EJ. Overlapping fragments of REJ
cDNA were cloned (total length, 5,596 bp). The sequence was confirmed by microsequencing six
peptides of mature REJ and by Western blotting with antibody to a synthetic
peptide designed from the sequence. Complete deglycosylation of REJ followed by Western blotting yielded a size estimate in agreement with that of the mature amino acid sequence. REJ is modular in design; it contains one
EGF module and two
C-type lectin carbohydrate-recognition modules. Most importantly, it contains a novel module, herein named the REJ module (700 residues), which shares extensive homology with the human
polycystic kidney disease protein (PKD1). Mutations in PKD1 cause
autosomal dominant polycystic kidney disease, one of the most frequent
genetic disease of humans. The lesion in cellular physiology resulting from mutations in the
PKD1 protein remains unknown. The homology between REJ modules of the sea urchin REJ and human PKD1 suggests that PKD1 could be involved in ionic regulation.