Phosphoinositides and Phosphoinositide 3-kinase

Phosphatidylinositol and its phosphorylated derivatives play crucial roles in a broad range of signal transduction processes. To shed light on processes that lead to the formation of phosphoinositide-enriched microdomains, mixed monolayers were investigated. DOPtdIns is capable to mix partially with condensed DSPC and to form mixed crystals which differ significantly from those formed by pure DSPC. DOPtdIns(4,5)P2 in mixtures with DSPC is to a much larger extent phase separated (see Figure below). In biological systems, an enzymatic phosphorylation of phosphatidylinositol in mixed domains may cause their insolubility in ordered PC areas and lead to a cooperative reorganization of the host lipid membrane.

The recruitment of phosphoinositide 3-kinase γ (PI3Kγ) to the membrane is the crucial requirement for the initiation of, e.g., inflammation cascades by second messenger production (cooperation with Prof. R. Klinger, University of Jena). The adsorption behavior of GST-PI3Kγ to non-substrate as well as to substrate lipids was investigated by IRRAS. The enzyme does not interact with condensed zwitterionic or anionic monolayers. However, it can penetrate into uncompressed fluid monolayers. The most important finding is that the protein affinity for the monolayer surface is considerably increased when the lipid has an anionic head group and contains an arachidonoyl fatty acyl chain in sn-2 position. The protein adsorption has a condensing effect in phosphoinositide monolayers.