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© The Rockefeller University Press, 0021-9525/1998//107 $5.00
The Journal of Cell Biology, Volume 143, Number 1, , 1998 107-119


Regular Articles

Small Espin: A Third Actin-bundling Protein and Potential Forked Protein Ortholog in Brush Border Microvilli



James R. Bartles, Lili Zheng, Anli Li, Allison Wierda, and Bin Chen

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 {alpha}-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.

Key Words: cytoskeleton • actins • microvilli • intestines • kidney



Address all correspondence to James R. Bartles, Department of Cell and Molecular Biology, Northwestern University Medical School, 303 East Chicago Avenue, Chicago, IL 60611. Tel.: (312) 503-1545. Fax: (312) 503-7912. E-mail: j-bartles{at}nwu.edu

1. Abbreviation used in this paper: GFP, green fluorescent protein.

2. We have discovered three errors in the sequence of the large isoform between nucleotides 1835 and 1907 (Bartles et al., 1996). These errors have been corrected in the database (available from GenBank/EMBL/ DDBJ under accession number U46007). The corrections bring about a shift in reading frame over a span of 22 amino acids and result in the addition of one amino acid. The corrected total number of amino acids for the large isoform is 837, and the corrected amino acid sequence for this region is: 607-GAGAACGQRRSSSSTGSTKSFNMMSPTG-634.



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