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J. Cell Biol.,
Volume 143, Number 1, October 5, 1998 107-119
Department of Cell and Molecular Biology, Northwestern University Medical School, Chicago, Illinois 60611
An ~30-kD isoform of the actin-binding/
bundling protein espin has been discovered in the brush
borders of absorptive epithelial cells in rat intestine and
kidney. Small espin is identical in sequence to the
COOH terminus of the larger (~110-kD) espin isoform identified in the actin bundles of Sertoli cell-spermatid
junctional plaques (Bartles, J.R., A. Wierda, and
L. Zheng. 1996. J. Cell Sci. 109:1229-1239), but it contains two unique peptides at its NH2 terminus. Small espin was localized to the parallel actin bundles of brush
border microvilli, resisted extraction with Triton X-100,
and accumulated in the brush border during enterocyte
differentiation/migration along the crypt-villus axis in
adults. In transfected BHK fibroblasts, green fluorescent protein-small espin decorated F-actin-containing fibers and appeared to elicit their accumulation and/or
bundling. Recombinant small espin bound to skeletal
muscle and nonmuscle F-actin with high affinity (Kd = 150 and 50 nM) and cross-linked the filaments into bundles. Sedimentation, gel filtration, and circular dichroism analyses suggested that recombinant small espin was a monomer with an asymmetrical shape and a high
percentage of
-helix. Deletion mutagenesis suggested
that small espin contained two actin-binding sites in its
COOH-terminal 116-amino acid peptide and that the
NH2-terminal half of its forked homology peptide was
necessary for bundling activity.
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