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

The espins are actin-binding and -bundling proteins localized to parallel a ctin bundles. The 837-amino-acid "espin" of Sertoli cell-spermatid junction s (ectoplasmic specializations) and the 253-amino-acid "small espin" of bru sh 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-bindi ng and -bundling properties of espin constructs and the stoichiometry and d evelopmental accumulation of espin within the ectoplasmic specialization. A n espin construct bound to F-actin with an approximately threefold higher a ffinity (K-d = similar to 70 nM) than small espin and was similar to 2.5 ti mes more efficient at forming bundles. The increased affinity appeared to b e 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 similar to 1 m u M, decorated actin stress fiber-like structures in transfected cells, and was mapped to a peptide between the two proline-rich peptides in the N ter minus of espin. Espin was detected at similar to 4-5 x 10(6) copies per ect oplasmic specialization, or similar to 1 espin per 20 actin monomers and ac cumulated there coincident with the formation of parallel actin bundles dur ing spermiogenesis. These results suggest that espin is a major actin-bundl ing protein of the Sertoli cell-spermatid ectoplasmic specialization.