In search of molecular dovetails in mussel byssus: from the threads to the stem.

We recently described the cDNA sequence for a unique collagenous protein, preCol-P, in the byssal threads of the marine mussel Mytilus edulis. The translated amino acid sequence encodes an unprecedented block-copolymer-like sequence with a central collagenous domain flanked by elastin-like sequences. Here, we report on the presence of two additional variants of preCol-P. The distribution of these variants in M. edulis foot tissue was examined by reverse transcription followed by polymerase chain reaction (RT-PCR) and in situ hybridization techniques. One of the variants, P33, exhibits a graded distribution with decreasing concentrations along the longitudinal axis of the foot. The second variant, P22, is expressed only at the base of the mussel's foot. In situ hybridization confirms the exclusive expression of preCol-P variant P22 in the stem gland. We propose that this variant may represent a molecular 'dovetail' between the proximal thread and the byssal stem, imparting extensibility and elastic recoil to the ring portion of the stem.