The Molecular Mass and Isoelectric Point of Plant Proteomes

Abstract

12A proteomic analysis of proteomes from 145 plant species revealed a pI range of 1.99 (epsin) 13 to 13.96 (hypothetical protein). The molecular mass of the plant proteins ranged from 0.54 to 14 2236.8 kDa. A putative Type-I polyketide synthase (22244 amino acids) in Volvox carteri was 15 found to be the largest protein in the plant kingdom and was not found in higher plant species. 16 Titin (806.46 kDa) and misin/midasin (730.02 kDa) were the largest proteins identified in 17 higher plant species. The pI and molecular weight of the plant proteome exhibited a trimodal 18 distribution. An acidic pI (56.44% of proteins) was found to be predominant over a basic pI 19 (43.34% of proteins) and the abundance of acidic pI proteins was higher in unicellular algae 20 species relative to multicellular higher plants. In contrast, the seaweed, Porphyra umbilicalis, 21 possesses a higher proportion of basic pI proteins (70.09%). Plant proteomes were also found 22 to contain the amino acid, selenocysteine (Sec), which is the first report of the presence of this 23 amino acid in plants. Additionally, plant proteomes also possess ambiguous amino acids Xaa 24 (unknown), Asx (asparagine or aspartic acid), Glx (glutamine or glutamic acid), and Xle 25 (leucine or isoleucine) as well.26 27 Pyrrolysine 29 30 31 32 33The isoelectric or isoionic point of a protein is the pH at which a protein carries no net electrical 34 charge and hence is considered neutral 1-4 . The zwitterion form of a protein becomes dominant 35 at neutral pH. The pI of polypeptides is largely dependent on the dissociation constant of the 36 ionisable groups 5 . The major ionisable groups present in the amino acids are arginine, 37 aspartate, cysteine, histidine, glutamate, lysine, and glutamate, where they play a major role in 38 determining the pI of a protein 6-8 . Co-translational and post-translational modifications of a 39 protein, however, can also play a significant role in determining the pI of a protein 9,10 . The 40 exposure of charged residues to the solvents, hydrogen bonds (diploe interactions) and 41 dehydration also impact the pI of a protein 11,12 . The inherent pI of protein, however, is 42 primarily based on its native protein sequence. The pI of a protein is crucial to understanding 43 its biochemical function and thus determining pI is an essential aspect of proteomic studies. 44During electrophoresis, the direction of movement of a protein in a gel or other matrix depends 45 its'pI, hence numerous proteins can be separated based on their pI [13][14][15][16] . Given the impact of 46 post-translational modifications and other biochemical alterations (phosphorylation, 47 49 9,17,18. Nonetheless, an estimated isoelectric point is highly important and a commonly 50 identified parameter. 51Several studies have been conducted to understand the pI of proteins/polypeptides 3,19-21 . These 52 studies have been mainly based on animal, bacteria, and virus models and databases containing 53 the pI of experimentally verified proteins. None of these da...