ABSTRACT
A differential display screen for transcripts that, like the GLUT4 glucose transporter, are expressed in a fat/muscle-specific manner led to the identification and cloning of PIKfyve, a novel mammalian protein of 2052 amino acids. It harbors several functional domains, including an N-terminal Zn2+-binding FYVE finger, a chaperonin-like region in the middle of the molecule and a C-terminal phosphoinositide (PI) kinase homology. Intracellular PIKfyve partitions in both particulate and soluble fractions, and localizes on peripheral vesicular structures distinct from the recycling endosomes and GLUT4containing membranes. In in vitro assays PIKfyve displays the highest substrate specificity for natural PtdIns and generates PtdIns 5-P as well as PtdIns 3, 5-P2, perhaps from PtdIns 3-P present as an impurity in the PtdIns substrate preparation or generated in situ by associated PI 3-kinase(s). In 3T3-L1 adipocytes, which appear to express the highest levels of the PIKfyve message and protein, PIKfyve is found associated with insulin-stimulated, wortmannin-sensitive PI 3-kinase. Potential roles of PIKfyve and its lipid products, the newly identified PtdIns 5-P and PtdIns 3, 5-P2, in cell signaling and membrane trafficking events are discussed.
