Arranged marriage in lipid signalling? The limited choices of PtdIns(4,5)P2 in finding the right partner

Inositol-containing phospholipids (phosphoinositides, PIs) control numerous cellular processes in eukaryotic cells. For plants, a key involvement of PIs has been demonstrated in the regulation of membrane trafficking, cytoskeletal dynamics and in processes mediating the adaptation to changing environmental conditions. Phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P-2) mediates its cellular functions via binding to various alternative target proteins. Such downstream targets of PtdIns(4,5)P-2 are characterised by the possession of specific lipid-binding domains, and binding of the PtdIns(4,5)P-2 ligand exerts effects on their activity or localisation. The large number of potential alternative binding partners - and associated cellular processes - raises the question how alternative or even contrapuntal effects of PtdIns(4,5)P-2 are orchestrated to enable cellular function. This article aims to provide an overview of recent insights and new views on how distinct functional pools of PtdIns(4,5)P-2 are generated and maintained. The emerging picture suggests that PtdIns(4,5)P-2 species containing different fatty acids influence the lateral mobility of the lipids in the membrane, possibly enabling specific interactions of PtdIns(4,5)P-2 pools with certain downstream targets. PtdIns(4,5)P-2 pools with certain functions might also be defined by protein-protein interactions of PI4P 5-kinases, which pass PtdIns(4,5)P-2 only to certain downstream partners. Individually or in combination, PtdIns(4,5)P-2 species and specific protein-protein interactions of PI4P 5-kinases might contribute to the channelling of PtdIns(4,5)P-2 signals towards specific functional effects. The dynamic nature of PI-dependent signalling complexes with specific functions is an added challenge for future studies of plant PI signalling.