Two novel phosphatidylinositol-4-phosphate 5-kinase type Iγ splice variants expressed in human cells display distinctive cellular targeting

Abstract

The generation of various phosphoinositide messenger molecules at distinct locations within the cell is mediated via the specific targeting of different isoforms and splice variants of phosphoinositide kinases. The lipid messenger PtdIns(4,5)P2 is generated by several of these enzymes when targeted to distinct cellular compartments. Several splice variants of the type Iγ isoform of PIPK (PtdIns4P 5-kinase), which generate PtdIns(4,5)P2, have been identified, and each splice variant is thought to serve a unique functional role within cells. Here, we have identified two novel C-terminal splice variants of PIPKIγ in human cells consisting of 700 and 707 amino acids. These two splice variants are expressed in multiple tissue types and display PIPK activity in vitro. Interestingly, both of these novel splice variants display distinct subcellular targeting. With the addition of these two new splice isoforms, there are minimally five PIPKIγ splice variants that have been identified in mammals. Therefore, we propose the use of the HUGO (Human Genome Organization) nomenclature in the naming of the splice isoforms. PIPKIγ_i4 (700 amino acids) is present in the nucleus, a targeting pattern that has not been previously observed in any PIPKIγ splice variant. PIPKIγ_i5 (707 amino acids) is targeted to intracellular vesicle-like structures, where it co-localizes with markers of several types of endosomal compartments. As occurs with other PIPKIγ splice variants, the distinctive C-terminal sequences of PIPKIγ_i4 and PIPKIγ_i5 may facilitate association with unique protein targeting factors, thereby localizing the kinases to their appropriate cellular subdomains for the site-specific generation of PtdIns(4,5)P2.

PIPKIγi5 regulates the endosomal trafficking and degradation of E-cadherin

“…PIPKIci2 is also regulated by tyrosine phosphorylation downstream of growth factor receptors . The unique C-terminus of PIPKIci5 contains a tyrosine motif similar to the Src phosphorylation site in PIPKIci2 (Schill and Anderson, 2009b) (Fig. 4C).…”

“…These findings position PIPKIc as a crucial regulator of the assembly of cell-cell contacts and the intracellular transport of components of these complexes. Recently, PIPKIci5 was found to localize to endosomes and interact with the PI(4,5)P 2 effector SNX5 to regulate the lysosomal degradation of the epidermal growth factor receptor (EGFR) (Schill and Anderson, 2009b;Sun et al, 2013). Because Ecadherin binds to the conserved region that is present in PIPKIci1, i2, i4 and i5, here, we examined the role of PIPKIci5 in the lysosomal degradation of E-cadherin.…”

“…The i1 N-terminal sequence is conserved in PIPKIci2, i4 and i5. These three isoforms each contain a unique peptide sequence at the C-terminal end, which mediates specific localization and function of each isoform (Schill and Anderson, 2009b). With the recent discovery of PIPKIci1-i6 in mammalian cells, here, we use the HUGO nomenclature for splice variants (PIPKIc87 or PIPKIc640 is referred to as PIPKIci1, PIPKIc90 or PIPKIc668 is referred to as PIPKIci2) (Schill and Anderson, 2009b;Xia et al, 2011).…”

“…Additionally, several C-terminal splice variants of PIPKIc (PIPKIci1, i2, i4 and i5) have been identified in mammalian cells, and these splice variants have specific functions (Bairstow et al, 2006;Di Paolo et al, 2002;Giudici et al, 2004;Ling et al, 2007;Ling et al, 2002;Schill and Anderson, 2009b;Sun et al, 2007;Xia et al, 2011). PIPKIci1 consists of 640 amino acids and is localized to the plasma membrane and cytoplasm.…”

Phosphatidylinositol Phosphate 5-Kinase Iγ and Phosphoinositide 3-Kinase/Akt Signaling Couple to Promote Oncogenic Growth

“…PIPKI␣ is predominantly localized to the nucleus and controls the nuclear events, whereas PIPKI␤ is targeted to perinuclear regions/endosomes (9,10). PIPKI␥ is expressed as at least six splice variants (PIPKI␥i1, PIPKI␥i2, PIPKI␥i3, PIPKI␥i4, PIPKI␥i5, and PIPKI␥i6) in mammalian cells (11,12); among them, PIPKI␥i2 can be targeted to the cell-matrix interface via an interaction with talin and plays role in focal adhesion assembly (13)(14)(15), whereas PIPKI␥i4 and PIPKI␥i5 are localized to the nucleus and endosomes, respectively (11,16). Src phosphorylation of PIPKI␥i2 regulates its interaction with talin and its targeting to focal adhesion sites (17).…”