Mammalian cells contain two isoforms of the type II PI4K (phosphoinositol 4-kinase), PI4KIIα and β. These 55 kDa proteins have highly diverse N-terminal regions (approximately residues 1–90) but conserved catalytic domains (approximately from residue 91 to the C-termini). Nearly the entire pool of PI4KIIα behaves as an integral membrane protein, in spite of a lack of a transmembrane domain. This integral association with membranes is due to palmitoylation of a cysteine-rich motif, CCPCC, located within the catalytic domain. Although the CCPCC motif is conserved in PI4KIIβ, only 50% of PI4KIIβ is membrane-associated, and approximately half of this pool is only peripherally attached to the membranes. Growth factor stimulation or overexpression of a constitutively active Rac mutant induces the translocation of a portion of cytosolic PI4KIIβ to plasma membrane ruffles and stimulates its activity. Here, we demonstrate that membrane-associated PI4KIIβ undergoes two modifications, palmitoylation and phosphorylation. The cytosolic pool of PI4KIIβ is not palmitoylated and has much lower lipid kinase activity than the membrane-associated kinase. Although only membrane-associated PI4KIIβ is phosphorylated in the unique N-terminal region, this modification apparently does not influence its membrane binding or activity. A series of truncation mutants and α/β chimaeras were generated to identify regions responsible for the isoform-specific behaviour of the kinases. Surprisingly, the C-terminal approx. 160 residues, and not the diverse N-terminal regions, contain the sites that are most important in determining the different solubilities, palmitoylation states and stimulus-dependent redistributions of PI4KIIα and β.

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