The phosphatidylinositol transfer protein RdgBβ binds 14-3-3 via its unstructured C-terminus, whereas its lipid-binding domain interacts with the integral membrane protein ATRAP (angiotensin II type I receptor-associated protein)

PITPs [PI (phosphatidylinositol) transfer proteins] bind and transfer PI between intracellular membranes and participate in many cellular processes including signalling, lipid metabolism and membrane traffic. The largely uncharacterized PITP RdgB beta (PITPNC1; retinal degeneration type B beta), contains a long C-terminal disordered region following its defining N-terminal PITP domain. In the present study we report that the C-terminus contains two tandem phosphorylated binding sites (Ser(274) and Ser(299)) for 14-3-3. The C-terminus also contains PEST sequences which are shielded by 14-3-3 binding. Like many proteins containing PEST sequences, the levels of RdgB beta are regulated by proteolysis. RdgB beta is degraded with a half-life of 4 h following ubiquitination via the proteasome. A mutant RdgB beta which is unable to bind 14-3-3 is degraded even faster with a half-life of 2 h. In vitro, RdgB beta is 100-fold less active than PITP alpha for PI transfer, and RdgB beta proteins (wild-type and a mutant that cannot bind 14-3-3) expressed in COS-7 cells or endogenous proteins from heart cytosol do not exhibit transfer activity. When cells are treated with PMA, the PITP domain of RdgB beta interacts with the integral membrane protein ATRAP (angiotensin II type I receptor-associated protein; also known as AGTRAP) causing membrane recruitment. We suggest that RdgB beta executes its function following recruitment to membranes via its PITP domain and the C-terminal end of the protein could regulate entry to the hydrophobic cavity.