Phosphoinositides have a pivotal role as precursors to important second messengers and as bona fide signaling and scaffold targeting molecules. Phosphatidylinositol 4-kinases (PtdIns 4-kinases or PI4Ks) are at the apex of the phosphoinsitide cascade. Sequence analysis revealed that mammalian cells contain two type II PtdIns 4-kinase isoforms, now termed PI4KIIα and PI4KIIβ. PI4KIIα was cloned first. It is tightly membrane-associated and behaves as an integral membrane protein. In this study, we cloned PI4KIIβ and compared the two isoforms by monitoring the distribution of endogenous and overexpressed proteins, their modes of association with membranes, their response to growth factor stimulation or Rac-GTP activation, and their kinetic properties. We find that the two kinases have different properties. PI4KIIβ is primarily cytosolic, and it associates peripherally with plasma membranes, endoplasmic reticulum, and the Golgi. In contrast, PI4KIIα is primarily Golgi-associated. Platelet-derived growth factor promotes PI4KIIβ recruitment to membrane ruffles. This effect is potentially mediated through Rac; overexpression of the constitutively active RacV12 induces membrane ruffling, increases PI4KIIβ translocation to the plasma membrane, and stimulates its activity. The dominant-negative RacN17 blocks plasma membrane association and inhibits activity. RacV12 does not boost the catalytic activity of PI4KIIα further, probably because it is constitutively membrane-bound and already activated. Membrane recruitment is an important mechanism for PI4KIIβ activation, because microsome-bound PI4KIIβ is 16 times more active than cytosolic PI4KIIβ. Membrane-associated PI4KIIβ is as active as membrane-associated PI4KIIα and has essentially identical kinetic properties. We conclude that PI4KIIα and PI4KIIβ may have partially overlapping, but not identical, functions. PI4KIIβ is activated strongly by membrane association to stimulate phosphatidylinositol 4,5-bisphosphate synthesis at the plasma membrane. These findings provide new insight into how phosphoinositide cascades are propagated in cells.
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