ABSTRACT
Substitution Rate .......................................................................... 270 10.2.6 Detecting Selection in the Charged Residues .................. 271 10.3 Results and Discussion ................................................................ 271 10.3.1 Calculation of pI .............................................................. 271 10.3.2 Are the Shifts in the pI of Some Milk Proteins Important
Compared to Whole Proteome Comparison? .............................. 275 10.3.3 Differences in Length Between Orthologs due to
Insertions or Deletions are Associated with the pIshift in Certain Proteins ............................................................................ 276
10.3.4 Selection Causing pI Change ........................................... 277
10.3.5 Selection Pressures for Changes in pI: the Roles of Dietary, Morphological, and Intrinsic Milk Protein Factors ........ 280
10.3.6 Phosphorylation and Glycosylation .................................. 282 10.4 Conclusion ................................................................................... 283 Keywords .............................................................................................. 284 Acknowledgment .................................................................................. 285 Competing Interests .............................................................................. 285 Authors’ Contributions .......................................................................... 285 References ............................................................................................. 285 Credits ................................................................................................... 287
10.1 INTRODUCTION
The isoelectric point (pI) and charge of a protein is important for solubility, subcellular localization, and interaction. There is a correlation between subcellular location and protein pI [1, 2]. Proteins in the cytoplasm possess an acidic pI (pI < 7.4), while those in the nucleus have a more neutral pI (7.4 < pI < 8.1) [1, 2]. It has also been shown that the pI can vary greatly, depending on both insertion and deletions between orthologs, and the ecology of the organism [3]. Kirga et al. [3] have shown that the pI of membrane proteins of bacteria correlates with their ecological niche, and changes dramatically from acidic to basic. For example, some prokaryotes that infect human have a pI that reflects their localization in the human body, compensating for the pH change. E. coli that resides in the intestines has more acidic proteins, and H. pylori that infects the acidic stomach has more negatively charged proteins [3].
