Figure 4.

A model illustrating the compartmentalization of phosphoinositide signaling in the nucleus. Current data suggests two compartments for the nuclear phosphoinositide cycle: One associated with the nuclear envelope and another in a subnuclear compartment separate from known membrane structures. In both compartments, PI is sequentially phosphorylated by PI kinases (PIK) and PIP kinases (PIPK) to generate PIP₂, which could then be metabolized by PLC to generate IP₃ and then higher inositol phosphates (IP_n) or phosphorylated by PI 3-kinase creating PIP₃. In subnuclear compartments, phosphoinositides are hypothesized to be associated with carrier or effector proteins. Such proteins could be specific for certain functions and/or could present phosphoinositides to other effectors. In addition, regulation of nuclear actin polymerization and actin binding proteins, such as N-WASP, CapZ and ADF (actin/cofilin depolymerising factor), either from envelope-bound or endonuclear phosphoinositides (shown by dashed arrows) has been shown to affect many aspects of gene expression.