Phosphatidylinositol transfer protein (PITP) or priming in exocytosis protein 3 (PEP3) is a ubiquitous cytosolic domain involved in transport of

phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...

s from their site of synthesis in the

endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...

and Golgi to other

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

Biological function

PITP has been also shown to be an essential component of the polyphosphoinositide synthesis machinery and is hence required for proper signalling by

epidermal growth factor Epidermal growth factor (EGF) is a protein that stimulates cell growth and differentiation by binding to its receptor, EGFR. Human EGF is 6-k Da and has 53 amino acid residues and three intramolecular disulfide bonds. EGF was originally descr ...

and f-Met-Leu-Phe, as well as for

exocytosis Exocytosis () is a form of active transport and bulk transport in which a cell transports molecules (e.g., neurotransmitters and proteins) out of the cell ('' exo-'' + ''cytosis''). As an active transport mechanism, exocytosis requires the use o ...

. The role of PITP in polyphosphoinositide synthesis may also explain its involvement in intracellular vesicular traffic.

Structure and evolution

Along with the structurally unrelated Sec14p family (found in ), this family can bind/exchange one molecule of phosphatidylinositol (PI) or phosphatidylcholine (PC) and thus aids their transfer between different membrane compartments. There are three sub-families - all share an N-terminal PITP-like domain, whose sequence is highly conserved. It is described as consisting of three regions. The N-terminal region is thought to bind the lipid and contains two helices and an eight-stranded, mostly antiparallel beta-sheet. An intervening loop region, which is thought to play a role in protein-protein interactions, separates this from the C-terminal region, which exhibits the greatest sequence variation and may be involved in membrane binding. This motif marks PITP as part of the larger SRPBCC (

START Start can refer to multiple topics: *Takeoff, the phase of flight where an aircraft transitions from moving along the ground to flying through the air * Starting lineup in sports *Standing start, and rolling start, in an auto race Acronyms *St ...

/RHOalphaC/PITP/ Bet v1/CoxG/CalC) domain superfamily. PITP alpha (

UniProt UniProt is a freely accessible database of protein sequence and functional information, many entries being derived from genome sequencing projects. It contains a large amount of information about the biological function of proteins derived from ...

) has a 16-fold greater affinity for PI than PC. Together with PITP beta (UniProt ), it is expressed ubiquitously in all tissues.

Human proteins

The family of human phosphatidylinositol transfer proteins has several members: * Phosphatidylinositol transfer protein, alpha (PITPNA) * Phosphatidylinositol transfer protein, beta (PITPNB) * Phosphatidylinositol transfer protein, cytoplasmic 1 (PITPNC) * Phosphatidylinositol transfer protein, membrane-associated 1 (PITPNM1) * Phosphatidylinositol transfer protein, membrane-associated 2 (PITPNM2) * PITPNM family member 3 (PITPNM3)

References

Protein domains Protein families Peripheral membrane proteins {{membrane-protein-stub