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)P(2) is generated by several of these enzymes when targeted to distinct cellular compartments. Several splice variants of the type Igamma isoform of PIPK (PtdIns4P 5-kinase), which generate PtdIns(4,5)P(2), 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 PIPKIgamma 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 PIPKIgamma 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. PIPKIgamma_i4 (700 amino acids) is present in the nucleus, a targeting pattern that has not been previously observed in any PIPKIgamma splice variant. PIPKIgamma_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 PIPKIgamma splice variants, the distinctive C-terminal sequences of PIPKIgamma_i4 and PIPKIgamma_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)P(2).