Ajit,

At the isoelectric point proteins are more likely going to come into contact with one another, because repulsive Coulombic  interactions are at a minimum.   Examine what happens when you add lemon juice to milk. Prior to adding it, milk is a stable colloidal mixture.  Afterwards, the proteins clump together and precipitate out of solution. 

John

I also want to mention that the theoretical pI that you obtain from simple amino acid sequence of the protein might not represent the true picture of the protein; especially with membrane proteins, when they are locating next to many ionic lipid headgroups in the membranes.

At its isoelectric point (pI), the protein has no net charge because the positive and negative charges are equal. The theoretical pI is based on the primary sequence, but is unlikely to match the actual pI because some of the charged side chains are either buried or in salt bridges. Also, the local environment of charged side chains can affect their pKa, which is what goes into the calculation of pI.

I don't think there is much significance to the pI for most proteins in vivo, when any single protein is usually not highly concentrated and therefore not likely to self-aggregate at its pI (some exceptions: albumin in blood and hemoglobin in red blood cells).

Thanks all of you for giving nice information. we are predicting bioinformatically various properties of our vaccine candidate