What is SR2 homology?

When a protein contains a src homology domain (SH2), the domain will bind to a phosphorylated tyrosyl residue and a few R-groups on another protein.  If the tyrosyl group of the other protein is not phosphorylated, the protein with SH2 will not bind.

It is important to realize that the SH2 will not bind to any phosphorylated tyrosyl.  It only binds to the phosphorylated tyrosyl groups that are surrounded by the correct amino acid R-groups.

Homology refers to the likeness in structure between domains, most likely due to evolutionary differentiation from a primordial protein.  That is, scientists first found one protein that would bind to tyrosyl residues when the tyrosyl residues were phosphorylated.  They named the binding domain of this protein Src.  Later, they found other proteins that would bind to phosphorylated tyrosyl residues and that contained a similar binding domain.  The binding domains were homologous to the original Src domain.

What is Pleckstrin homology

Pleckstrin homology(PH) domains are small domains that occur in a large variety of proteins. The domains are best known for their ability to target cellular membranes by binding specifically to phosphoinositides.  Often, proteins with PH homologies need help in binding to phosphoinositides but we do not cover that in this course.

What good is localizing an enzyme near the cell membrane?  The slow part of most cellular reactions is the time it takes for the reactants to bump into one another.  If the two reactants are attached to the cell membrane but free to diffuse along the membrane, the reaction happens much faster than if the two reactants were diffusing in the cytosol.