The Analysis of Solanum lycopersicum Sap Dark Proteome Reveals Ordered and Disordered Protein Abundance

Protein identity and functional roles within the cell provide the landscape of proteomics and other high-throughput technologies. However, not all protein sequences are cataloged with an identity or a functional protein family. The lack of identity and functional role of a set of proteins are collectively named as the dark proteome. Key structural features are, for example, ordered sequences (with a defined structural arrangement) and disordered sequences (presenting one or more intrinsically disordered stretches). Here, we reanalyzed eight proteomic datasets and the subset of the "unknown" proteome of S. lycopersicum to describe if there is a relationship between disorder, length, and tissue-specific abundance of proteins with key structural features in the relation of ordered/disordered abundance in the protein sequences. Intriguingly, we unveil that from the S. lycopersicum proteome, the "unknown" subset represents around 10% only. We further cataloged dark proteome in terms of ordered and disordered sequences and found that proteins with disorder represent around 23% of the total "unknown" proteins. Also, we describe an amino acid composition and sequence length enrichment both, in the ordered and disordered fraction of the dark proteome. Finally, we describe that proteins within the dark proteome can be related to a specific location and abundance in an organ or tissue. An unknown protein sequence presenting a combination of specific length and degree of disorder can be explored with other biotechnological alternatives to improve responses or tolerate abiotic stress, also serving as sensors during development or ripening stages. These findings suggest an opportunity to study "protein darkness" in terms of disorder and functional associations.