Espin contains an additional actin-binding site in its N terminus and is a major actin-bundling protein of the sertoli cell-spermatid ectoplasmic specialization junctional plaque

The espins are actin-binding and -bundling proteins localized to parallel actin bundles. The 837-amino-acid 'espin' of Sertoli cell-spermatid junctions (ectoplasmic specializations) and the 253-amino-acid 'small espin' of brush border microvilli are splice isoforms that share a C-terminal 116- amino-acid actin-bundling module but contain different N termini. To investigate the roles of espin and its extended N terminus, we examined the actin-binding and -bundling properties of espin constructs and the stoichiometry and developmental accumulation of espin within the ectoplasmic specialization. An espin construct bound to F-actin with an approximately threefold higher affinity (K(d) = ~70 nM) than small espin and was ~2.5 times more efficient at forming bundles. The increased affinity appeared to be due to an additional actin-binding site in the N terminus of espin. This additional actin-binding site bound to F-actin with a K(d) of ~1 μM, decorated actin stress fiber-like structures in transfected cells, and was mapped to a peptide between the two proline-rich peptides in the N terminus of espin. Espin was detected at ~4-5 x 10⁶ copies per ectoplasmic specialization, or ~1 espin per 20 actin monomers and accumulated there coincident with the formation of parallel actin bundles during spermiogenesis. These results suggest that espin is a major actin-bundling protein of the Sertoli cell-spermatid ectoplasmic specialization.