Sequence evolution, processing, and posttranslational modification of zonadhesin D domains in primates, as inferred...

Zonadhesin is a mammalian transmembrane sperm ligand. Precursor zonadhesin essentially consists of MAM (meprin/A5 antigen/mu receptor tyrosine phosphatase) domains, a mucin-like repeat, and D domains (homologous to von Willebrand D). Recent immunovisualization and binding assays indicate that zonadhesin D domains 1-3 bind postacrosomally to the zona pellucida. This feature has attracted considerable interest in the evolution of zonadhesin and its possible biological and biomedical implications. Previous molecular evolutionary analyses, however, were confined to cDNA sequences of only few distantly related species. Moreover, except for rabbit and pig, little is known about zonadhesin's processing. To delineate the situation in primates including humans, we analyze here the evolution of zonadhesin on the basis of D domain encoding cDNAs of about 4900 base pairs (bp) length from a representative primate sampling (1 Strepsirhini, 3 Cercopithecidae, 3 Platyrrhini, and human; 7 new sequences) plus GenBank data from mouse, rabbit, and pig. Site-specific (CODEML and HyPhy) analysis indicates positive evolution of zonadhesin. Moreover, moving window analysis (CRANN) points to a positive correlation of sequence evolution and sperm-competition. Significant accumulations of positively selected sites across interspecifically variable motifs (identified by PROSITE) suggest that positive selection promotes differences between species by amino acid exchanges and changes in posttranslational modification. In the case of zonadhesin D domains, positive selection might thus contribute to the species-specific binding of zonadhesin and zona pellucida. A high conservation of processing and dimerization motifs of primate zonadhesin in analogy to pig, on the other hand, illustrates that zonadhesin's backbone needs to meet basic requirements in order to retain function.