To identify species-specific regions of the sea urchin egg surface receptor for sperm, we cloned and sequenced the cDNA from S. franciscanus (Sf) ovary mRNA that is homologous to the S. purpuratus (Sp) sperm receptor sequence. The Sf cDNA contains an 886-amino-acid open reading frame (ORF) that is 96% identical at the nucleotide level to the Sp sperm receptor sequence over 2.9 kb. In contrast to the published Sp sequence, the Sf sequence does not encode a signal peptide or transmembrane domain. However, like the Sp sperm receptor sequence, the Sf protein has substantial similarity to the 70-kDa heat shock family of proteins and appears to encode a member of a newly identified subfamily of hsp70-related proteins designated hsp110/SSE. A BLAST search using the 5' end of the published Sp cDNA sequence as a query indicates that a segment of approximately 400 bp, which encodes the putative signal sequence, is 95% identical to Sp mitochondrial DNA. Resequencing of the Sp cDNA clone failed to confirm the presence of the published transmembrane and cytoplasmic domains. These results suggest that the published sequence of the Sp egg receptor for sperm may contain errors in the critical regions that were believed to encode an amino-terminal signal sequence, transmembrane domain, and cytoplasmic tail and that the protein products encoded by these cDNAs are highly related to mammalian cytoplasmic hsp110s.