A protein is formed with the combination of these acidic, basic (polar) and neutral (non-polar) amino acids. So, if the protein is containing more of basic amino acids it’ll be positively charged and if it’s containing more of acidic amino acids it’ll be negatively charged.
Above the isoelectric point, a protein carries a net negative charge—below it, a net positive charge.
A protein has its lowest solubility at its isoelectric point. If there is a charge at the protein surface, the protein prefers to interact with water, rather than with other protein molecules. This charge makes it more soluble. Without a net charge, protein–protein interactions and precipitation are more likely.