Immunoreceptor Signaling Proteins, Such as the TCR $\zeta$ Chain and CD3 Subunits, Have Conserved ITAM Motifs in Their

Immunoreceptor signaling proteins, such as the TCR $\zeta$ chain and CD3 subunits, have conserved ITAM motifs in their cytoplasmic tails. When fully phosphorylated, the ITAM recruits a tyrosine kinase with a tandem SH2 domain structure at the amino-terminal end of the protein. Tandem SH2 domain-containing kinases do not bind to sequences in other proteins, even if they contain a phosphorylated tyrosine because:

A) The amino acid sequence adjacent to the phosphorylated tyrosines in the ITAM motif is unique, and not found in any other proteins.
B) The affinity of a single SH2 domain within these kinases for a tyrosine phosphorylated sequence is too low for efficient binding.
C) The amino-terminal SH2 domain of the kinase has very high affinity for both of the phosphorylated tyrosines in the ITAM motif, so will not bind to other proteins.
D) The amino-terminal SH2 domain of the kinase is in an autoinhibited conformation and can only bind to a phosphorylated ITAM.
E) The tandem SH2 domain-containing kinase phosphorylates the tyrosines in the ITAM itself, so can only bind to these sequences.