Bifunctional peptides: targeting acidity and receptor transmembrane domains

Single spanning membrane receptors play important roles in development, disease, and homeostasis. However, many mechanistic details of receptor activation remain unknown. In particular the transmembrane domain is ultimately responsible for transmitting a signal across the cellular membrane without disrupting the membrane or changing its secondary structure. Receptors overcome this issue by forming oligomers, of which the transmembrane domain plays an integral role. Motifs within the transmembrane domain can drive oligomerization, or, when occluded, prevent it. A key question is how the transmembrane domain of receptors can induce specific activation only in certain cellular contexts. We address this question in four chapters. A review of recent literature demonstrates that receptors achieve specificity with combinations of mechanisms, including ligand-induced dimerization, rotation, and clustering (Chapter I). To determine what other factors may modulate dimerization, we characterized the transmembrane domain of an important receptor tyrosine kinase, EphA2. We find that this domain forms dimers, and this dimerization is reduced in the presence of a key second messenger lipid, phosphatidylinositol 4,5-bisphosphate (Chapter II). To modulate the dimerization of EphA2 in a cellular context, we then designed the receptor-targeting, pH-sensitive peptide, TYPE7 (Chapter III). TYPE7 binds to the EphA2 transmembrane domain, induces oligomerization, and activates the receptor. However, the EphA2 transmembrane domain contains two putative dimerization interfaces. We thus designed a suite of similar receptor-targeting, pH-responsive peptides to modulate the activity of the receptor (Chapter IV). We propose that the N-terminal portion of the transmembrane domain of EphA2 may be particularly robust and resistant to disruption.