The C-terminal half of Dpfp1 is dominated by the previously reported 13 amino acid consensus sequence: K-P-G-P-Y-D-Y-D-G-P-Y-D-K (Rzepecki and Waite, 1993).
MALDI-TOF analysis of native Dpfp1 indicates that the purified protein is represented by two major mass variants.
The primary structure of Dpfp1, deduced from overlapping cDNAs, represents the first complete sequence for a dreissenid byssal protein and an important advance in understanding the attachment strategy of the zebra mussel.
Table I Amino acid composition of deduced and native Dpfp1 Amino acid Native Deduced Asx 136.7 134.8 Thr 75.0 82.7 Ser 34.4 33.1 Glx 70.1 52.0 Pro 238.6 234.0 Gly 76.5 68.6 Ala 7.9 2.4 Val 50.4 52.0 Met 0.7 0.0 Ile 9.9 9.5 Leu 20.4 18.9 Dopa 66.6 N.D.
In previous studies, isoelectric focusing of purified Dpfp1 suggested the presence of at least 10 electrophoretic variants in the polymorphic family (Rzepecki and Waite, 1993).
This is especially true of proline, tyrosine, aspartic acid, lysine, threonine, and glycine residues, which together account for almost 75% of the amino acid composition of Dpfp1. The pattern of codon bias in compositionally dominant residues has also been noted in other marine byssal precursor proteins - notably Mcfp1 (Filpula et al., 1990), Mgfp1 (Inoue and Odo, 1994), Mcfp1 (Inoue et al., 1996b), and, to a lesser extent, Mgfp2 (Inoue et al., 1995) - and may reflect a need to express byssal structural proteins rapidly in response to developmental cues and changing environmental conditions.
More than 80% of the deduced primary amino acid sequence of Dpfp1 is composed of tandemly repeated and segregated motifs: one is a heptapeptide; the other, a tridecapeptide consensus motif that coincides with peptides sequenced previously (Rzepecki and Waite, 1993).
The N-terminal half of Dpfp1 is dominated by a heptapeptide motif that is repeated 22 times with some variation, particularly at position #7 of the consensus sequence.
The N-terminal half of Dpfp1 differs significantly from the C-terminal domain with its repeated 13 amino acid motif [ILLUSTRATION FOR FIGURE 4 OMITTED].