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) = approximately 70 nM) than small espin and was approximately 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 approximately 1 microM, 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 approximately 4-5 x 10(6) copies per ectoplasmic specialization, or approximately 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.
Chen
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B
Department of Cell and Molecular Biology, Northwestern University Medical School, Chicago, Illinois 60611, USA.
Li
A
A
Wang
D
D
Wang
M
M
Zheng
L
L
Bartles
J R
JR
D000199:Actins / Q000737:chemistry / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D008297:Male; D051379:Mice; D008840:Microfilament Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D016253:Microscopy, Immunoelectron; D011485:Protein Binding; D020033:Protein Isoforms / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D012671:Seminiferous Tubules / Q000378:metabolism / Q000648:ultrastructure; D012708:Sertoli Cells / Q000378:metabolism* / Q000648:ultrastructure; D013087:Spermatids / Q000378:metabolism*
D000199:Actins; D008840:Microfilament Proteins; D020033:Protein Isoforms; D011994:Recombinant Proteins; C104108:espin protein, mouse