Function of CD4 is all except

Ref Robbins 9/e p190-191 CD4 is a co-receptor of the T cell receptor (TCR) and assists the latter in communicating with antigen-presenting cells. The TCR complex and CD4 each bind to distinct regions of the antigen-presenting MHCII molecule - a1/b1 and b2, respectively. In CD4 the interaction involves its extracellular D1 domain. The resulting close proximity between the TCR complex and CD4 (extracellular and intracellular) allows the tyrosine kinase Lck bound to the cytoplasmic tail of CD4 to tyrosine-phosphorylate the Immunoreceptor tyrosine activation motifs (ITAM) on the cytoplasmic domains of CD3 to amplify the signal generated by the TCR. Lck is essential for the activation of many molecular components of the signaling cascade of an activated T cell. Depending on the signal, different types of T helper cells result. Phosphorylated ITAM motifs on CD3 recruit and activate SH2 domain-containing protein tyrosine kinases (PTK) such as Zap70 to fuher mediate downstream signalling through tyrosine phosphorylation, leading to transcription factor activation including NF-kB and consequent T cell activation.