A potential mediator of collagenous block copolymer gradients in mussel byssal threads

A potential mediator of collagenous block copolymer gradients in mussel byssal threads

Xiao-Xia Qin and
J. Herbert Waite†

Marine Biology/Biochemistry Program, University of Delaware, Newark, DE 19716

Edited by Jerome Gross, Massachussetts General Hospital, Charlestown, MA, and approved July 7, 1998 (received for review December 17, 1997)

Abstract

Mussel byssal threads contain unusual block copolymer-like proteins that combine collagen with flanking domains that resemble silk-fibroin (preCol-D) or elastin (preCol-P). These are distributed in complementary gradients along the length of the threads and as precursors in the mussel foot. We discuss a 76-kDa precursor, preCol-NG, from a cDNA library of the foot where it has no gradient but rather is distributed evenly along the distal to proximal axis. A pepsin-resistant fragment of preCol-NG has been confirmed in byssal threads. Like preCol-D and -P, this protein has a central collagenous domain, flanking domains, an acidic patch, and histidine-rich termini. The flanking domains of preCol-NG resemble the glycine-rich proteins of plant cell walls with tandem XGly_n repeats where X denotes alanine, leucine, or asparagine but not proline. Similarity with the (glycine–alanine) repeats and poly(alanine) runs of arthropod silks also exists. Based on available evidence, a model of preCol axial assembly is proposed in which preCol-NG functions as a mediator between preCol-D/-P molecules. This is consistent with the observed progression of mechanical properties in byssal threads.

collagen/glycine-rich cell wall proteins/byssus/Mytilus/nongradient distribution

Footnotes

↵ † To whom reprint requests should be addressed. e-mail: hwaite{at}udel.edu.
This paper was submitted directly (Track II) to the Proceedings Office.
Data deposition: The sequence reported in this paper has been deposited in the GenBank database (accession no. AF043944).
ABBREVIATIONS:

NG,

nongradient;

RT,

reverse transcriptase;

Col-D,

pepsin-resistant collagen from the distal portion of byssus;

Col-NG,

pepsin-resistant NG collagen from the byssus;

Col-P,

pepsin-resistant collagen from the proximal portion of byssus;

preCol-D,

precursor of Col-D;

preCol-NG,

precursor of Col-NG;

preCol-P,

precursor of Col-P