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J. Biol. Chem., Vol. 280, Issue 47, 39332-39336, November 25, 2005

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Mapping Chemical Gradients within and along a Fibrous Structural Tissue, Mussel Byssal Threads^*

ChengJun Sun1 and J. Herbert Waite

From the Molecular, Cellular, and Developmental Biology Department and the Chemistry and Biochemistry Department, University of California Santa Barbara, Santa Barbara, California 93106

The byssal thread of a mussel is an extraorganismic connective tissue that exhibits a striking end-to-end gradient in mechanical properties and thus provides a unique opportunity for studying how gradients are made. Mfp-1 (Mytilus foot protein-1) is a conspicuous component of the protective outer cuticle of byssal threads given its high 3,4-dihydroxyphenylalanine (Dopa) content at 10-15 mol %. Amino acid analysis of mfp-1 extracted from successive foot sections of Mytilus galloprovincialis reveals a post-translationally mediated gradient with highest Dopa levels present in mfp-1 from the accessory gland near the tip of the foot decreasing gradually toward the base. The Dopa content of successive segments of byssal threads decreases from the distal to the proximal end and thus reflects the trend of mfp-1 in the foot. Inductively coupled plasma analysis indicates that certain metal ions including iron follow the trend in Dopa along the thread. Energy-dispersive x-ray spectrometry showed that iron, when present, was concentrated in the cuticle of the threads but sparse in the core. The axial iron gradient appears most closely correlated with the Dopa gradient. The direct incubation of mussels and byssal threads in Fe³⁺ supplemented seawater showed that byssal threads are unable to sequester iron from the seawater. Instead, particulate/soluble iron is actively taken up by mussels during filter feeding and incorporated into byssal threads during their secretion. Our results suggest that mussels may exploit the interplay between Dopa and metals to tailor the different parts of threads for specific mechanical properties.

Received for publication, August 5, 2005 , and in revised form, September 14, 2005.

^* This research was supported by Grant R01 DE015415 from the National Institutes of Health and Grant NCC-1-02037 NASA University Research, Engineering and Technology Institute in Bio-Inspired Materials. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: MCD Biology Dept., University of California, Santa Barbara, CA 93106. Tel.: 805-893-5787; Fax: 805-893-7998; E-mail: sun{at}lifesci.ucsb.edu.

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