Lipid Binding Controls Dimerization of the Coat Protein p24 Transmembrane Helix

Coat protein (COP) I and COP II complexes are involved in the transport of proteins between the endoplasmic reticulum and the Golgi apparatus in eukaryotic cells. The formation of COP I/II complexes at membrane surfaces is an early step in vesicle formation and is mastered by p24, a type I transmembrane protein. Oligomerization of p24 monomers was suggested to be mediated and/or stabilized via interactions within the transmembrane domain, and the p24 transmembrane helix appears to selectively bind a single sphingomyelin C18:0 molecule. Furthermore, a potential cholesterol-binding sequence has also been predicted in the p24 transmembrane domain. Thus, sphingomyelin and/or cholesterol binding to the transmembrane domain might directly control the oligomeric state of p24 and, thus, COP vesicle formation. In this study, we show that sequence-specific dimerization of the p24 transmembrane helix is mediated by a LQ7 motif, with Gln187 being of special importance. Whereas cholesterol has no direct impact on p24 dimerization, binding of the sphingolipid can clearly control dimerization of p24 in rigid membrane regions. We suggest that specific binding of a sphingolipid to the p24 transmembrane helix affects p24 dimerization in membranes with increased cholesterol contents. A clearly defined p24 dimerization propensity likely is crucial for the p24 activity, which involves shuttling in between the endoplasmic reticulum and the Golgi membrane, in which cholesterol and SM C18:0 concentrations differ.