HIV-Tat Protein Forms Phosphoinositide-dependent Membrane Pores Implicated in Unconventional Protein Secretion

Abstract

Background: Unconventional secretion of both HIV-Tat and FGF2 depends on the phosphoinositide PI(4,5)P 2 . Results: HIV-Tat forms membrane-inserted oligomers concomitant with PI(4,5)P 2 -dependent membrane pore formation. Conclusion: HIV-Tat and FGF2 show similar properties with a tight correlation between membrane pore formation and unconventional secretion from cells. Significance: Evidence is provided that suggests a common mechanism of unconventional secretion with potential relevance for a broad range of cargoes.

The Startling Properties of Fibroblast Growth Factor 2: How to Exit Mammalian Cells without a Signal Peptide at Hand

“…Here, the role of the oligomer would be to act as a highly specific translocon mediating physical passage of FGF2 monomers across plasma membranes. Although not fully conclusive at this point, this model is supported by the recently observed tight association of FGF2 oligomers with membranes (21). The "translocon" model also eliminates the need of conversion of disulfide-linked FGF2 oligomers into monomers on the surface of plasma membranes, a process that would be difficult to explain considering the oxidative environment of the extracellular space.…”

“…Using reconstitution experiments, it has now been shown that HIV-Tat has properties similar to FGF2. These properties include PI(4,5)P 2 -dependent membrane binding, oligomerization, and membrane pore formation (21). Also, HIV-Tat has been demonstrated to bind to heparan sulfate proteoglycans (44) that may play a similar role in HIV-Tat secretion when compared with what is known for FGF2 (22,25).…”

Small Molecule Inhibitors Targeting Tec Kinase Block Unconventional Secretion of Fibroblast Growth Factor 2

“…FGF2 exits cells by direct translocation across the plasma membrane (18,19). This process involves (i) membrane recruitment at the inner leaflet mediated by the phosphoinositide PI(4,5)P 2 (1, 2, 21, 22), (ii) FGF2 oligomerization and membrane pore formation (9,11,23,24,30), and (iii) extracellular trapping mediated by membraneproximal heparan sulfate proteoglycans (25,26). In addition, Tec kinase (9, 31) and ATP1A1 (12,(32)(33)(34), two additional factors physically associated with the plasma membrane, have been shown to play critical roles in the unconventional secretory pathway of FGF2 (12).…”

“…Beyond their biomedical implications as lead compounds for drug development, the identified small molecule inhibitors corroborate the role of Tec kinase as a stimulatory component of the unconventional secretory pathway of FGF2. They further represent useful tool compounds to study the mechanism of unconventional secretory processes in general, in particular with regard to other cargoes, such as HIV-Tat (27)(28)(29)(30).…”

“…Hallmarks of this process are (i) FGF2 recruitment at the inner leaflet of the plasma membrane mediated by the phosphoinositide PI(4,5)P 2 2 (9,21,22), (ii) FGF2 oligomerization and membrane pore formation (11,12,23,24), and (iii) extracellular trapping of FGF2 mediated by membrane-proximal heparan sulfate proteoglycans (1,2,25,26). Recently, these hallmarks of FGF2 secretion have also been implicated in unconventional secretion of HIV-Tat (HIV trans-activator of transcription) (12,(27)(28)(29)(30). Based on a genome-wide RNAi screening approach, two additional factors physically associated with the plasma membrane have been identified to play a role in FGF2 secretion, Tec kinase (9,11,12,24,31) and ATP1A1 (12,(32)(33)(34), the ␣ subunit of the Na/K-ATPase (35).…”