Introduction: Both marine and freshwater mussels adhere to rocks via a proteinaceous 'anchor' (byssus), consisting of multiple threads that attach the organism to substrates through small adhesive pads. Given that mussels are able to adhere to a wide variety of surfaces in wet conditions, their mechanism of adhesion has been investigated as a potential source of bioinspired adhesives for medical applications. The byssal proteins of marine mussel have been extensively studied in Mytilids, where 3,4-dihydroxyphenylalanine (DOPA) has been shown to be largely responsible for adhesion and cohesion, and has served as the basis for numerous bioinspired adhesives and coatings. Although the byssi of marine and freshwater mussels are superficially similar, freshwater mussels contain only trace amounts of DOPA. Furthermore, in at least one freshwater species (zebra mussels), we recently demonstrated that there are major differences between its byssal proteins and those of marine mussels, suggesting that the mechanism of adhesion may also be different. Here we report on the use of transcriptomics paired with bottom-up proteomics to investigate the byssal proteins of the quagga mussel (Dreissena bugensis), an invasive freshwater mussel species that now dominates the North American Great Lakes. Materials and Methods: Quagga mussels were collected from the mouth of the St. Lawrence River at Kingston, Ontario and kept in circulating artificial freshwater. RNA was isolated from the foot (byssal secreting organ) of the mussel, and used to prepare a transcriptome library using next-generation sequencing (Illumina 2500) followed by assembly in Trinity. Byssal proteins were isolated as from freshly secreted material as previously described. Quagga mussel byssal proteins were digested and analyzed using LC-MS/MS; peptide spectra were matched to the foot transcriptome library to fingerprint the entire protein primary sequences using the proteomics software PEAKS7, followed by BLAST analysis to identify novel sequences. Results and Discussion: The assembled cDNA library of the quagga mussel foot transcriptome contains over 200,000 transcripts. By matching spectra of extracted byssal protein fragments to this library, full sequences of fifteen novel quagga mussel byssal proteins, named Dreissena bugensis foot proteins 4 to 18 (Dbfp4 - Dbfp18), and new sequence data for two previously observed byssal proteins Dbfp1 and Dbfp2. Theoretical masses of the newly discovered proteins range from 4.3 kDa to 21.6 kDa. Many of these proteins show polymorphism and contain repeated patterns, which is typical of byssal proteins, and several display homology to zebra mussel proteins. Conclusions: The fifteen new proteins described in this study represent a major expansion in knowledge of the quagga mussel byssus proteome, and are a critical step towards understanding adhesion in this freshwater mussel. Further studies are needed to determine the spatial distribution and roles of these proteins, which may serve as a new source of bioinspiration for wet adhesives.
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