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
B. Chen et al., 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, MOL BIOL CE, 10(12), 1999, pp. 4327-4339
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.