Phosphoinositides (PIPs) are ubiquitous regulators of signal transduction events in eukaryotic cells. PIPs are degraded by various enzymes, including PIP phosphatases. The integral membrane Sac1 phosphatases represent a major class of such enzymes. The central role of lipid phosphatases in regulating PIP homeostasis notwithstanding, the biological functions of Sac1-phosphatases remain poorly characterized. Herein, we demonstrate that functional ablation of the single murine Sac1 results in preimplantation lethality in the mouse and that Sac1 insufficiencies result in disorganization of mammalian Golgi membranes and mitotic defects characterized by multiple mechanically active spindles. Complementation experiments demonstrate mutant mammalian Sac1 proteins individually defective in either phosphoinositide phosphatase activity, or in recycling of the enzyme from the Golgi system back to the endoplasmic reticulum, are nonfunctional proteins in vivo. The data indicate Sac1 executes an essential household function in mammals that involves organization of both Golgi membranes and mitotic spindles and that both enzymatic activity and endoplasmic reticulum localization are important Sac1 functional properties. INTRODUCTIONSpatial and temporal regulation of intracellular signaling in eukaryotic cells involves the compartmentalization of membrane surfaces into discrete, albeit often transient, functional units. There are several biochemical strategies by which cells generate such units or domains. One well-established strategy uses the chemical diversity offered by phosphoinositides (PIPs), i.e., phosphorylated forms of phosphatidylinositol (PtdIns) (Majerus, 1997;Fruman et al., 1998;Strahl and Thorner, 2007). The chemical heterogeneity of individual PIP species permits the construction of chemically unique platforms on membrane surfaces that, in turn, recruit unique cohorts of proteins that drive specific biological reactions. Spatial and temporal control of such reactions requires a finely coordinated balance between the activities of the lipid kinases that produce PIPs and the activities of enzymes that degrade them. PIP turnover is catalyzed by two general classes of enzymes: phospholipases and PIP phosphatases.Although experimental scrutiny of the phospholipases has historically been more intense, recent demonstrations of the significant biological functions played by PTEN, the myotubularins, and synaptojanins highlight the general importance of PIP phosphatases (Wishart et al., 2001;Wishart and Dixon, 2002;Wenk and De Camilli, 2004).The SAC domain derives from the yeast Sac1 protein (ySac1; Cleves et al., 1989), and it represents a signature for PIP phosphatase catalytic activity . PIP phosphatases such as phosphatase and tensin homolog (mutated in multiple advanced cancers 1) (PTEN) (Maehama et al., 2001), synaptojanins (Cremona et al., 1999), and synaptojanin-like proteins (Srinivasan et al., 1997;Stolz et al., 1998) all harbor SAC domains. The prototypical member of this family, ySac1, catalyzes the dephosphoryla...

Palladin is an actin-associated protein that contains proline-rich motifs within its amino-terminal sequence that are similar to motifs found in zyxin, vinculin, and the Listeria protein ActA. These motifs are known to be potential binding sites for the Vasodilator-Stimulated Phosphoprotein (VASP). Here, we demonstrate that palladin is an additional direct binding partner for VASP, by using co-immunoprecipitation and blot overlay techniques with both endogenous palladin and recombinant myc-tagged palladin. These results show that VASP binds to full-length palladin and also to the amino-terminal half of palladin, where the polyproline motifs are located. Using a synthetic peptide array, two discrete binding sites for VASP were identified within palladin's proline-rich amino-terminal domain. Using double-label immunofluorescence staining of fully-spread and actively-spreading fibroblasts, the extent of co-localization of palladin and VASP was explored. These proteins were found to strongly co-localize along stress fibers, and partially co-localize in focal adhesions, lamellipodia, and focal complexes. These results suggest that the recently described actin-associated protein palladin may play an important role in recruiting VASP to sites of actin filament growth, anchorage, and crosslinking.

The outgrowth of neurites is a critical step in neuronal maturation, and it is well established that the actin cytoskeleton is involved in this process. Investigators from our laboratory recently described a novel protein named palladin, which has been shown to play an essential role in organizing the actin cytoskeleton in cultured fibroblasts. We investigated the expression of palladin in the developing rat brain by Western blot and found that the E18 brain contained a unique variant of palladin that is significantly smaller (approximately 85 kDa) than the common form found in other developing tissues (90-92 kDa). Because the expression of a tissue-specific isoform suggests the possibility of a cell type-specific function, we investigated the localization and function of palladin in cultured cortical neurons. Palladin was found preferentially targeted to the developing axon but not the dendrites and was strongly localized to the axonal growth cone. When palladin expression was attenuated by transfection with antisense constructs in both the B35 neuroblastoma cell line and in primary cortical neurons, a reduction in the expression of palladin resulted in a failure of neurite outgrowth. These results implicate palladin as a critical component of the developing nervous system, with an important role in axonal extension.

Palladin is an actin‐associated protein that has been suggested to play critical roles in establishing cell morphology and maintaining cytoskeletal organization in a wide variety of cell types. Palladin has been shown previously to bind directly to three different actin‐binding proteins vasodilator‐stimulated phosphoprotein (VASP), α‐actinin and ezrin, suggesting that it functions as an organizing unit that recruits actin‐regulatory proteins to specific subcellular sites. Palladin contains sequences resembling a motif known to bind profilin. Here, we demonstrate that palladin is a binding partner for profilin, interacting with profilin via a poly proline‐containing sequence in the amino‐terminal half of palladin. Double‐label immunofluorescence staining shows that palladin and profilin partially colocalize in actin‐rich structures in cultured astrocytes. Our results suggest that palladin may play an important role in recruiting profilin to sites of actin dynamics.