The I/LWEQ module superfamily is a class of actin-binding proteins that contains a conserved C-terminal actin-binding element known as the I/LWEQ module. I/LWEQ module proteins include the metazoan talins, the cellular slime mold talin homologues TalA and TalB, fungal Sla2p, and the metazoan Sla2 homologues Hip1 and Hip12 (Hip1R). These proteins possess a similar modular organization that includes an I/LWEQ module at their C-termini and either a FERM domain or an ENTH domain at their N-termini. As a result of this modular organization, I/LWEQ module proteins may serve as linkers between cellular compartments, such as the plasma membrane and the endocytic machinery, and the actin cytoskeleton. Previous studies have shown that I/LWEQ module proteins bind to F-actin. In this report, we have determined the affinity of the I/LWEQ module proteins Talin1, Talin2, huntingtin interacting protein-1 (Hip1), and the Hip1-related protein (Hip1R/Hip12) for F-actin and identified a conserved structural element that interferes with the actin binding capacity of these proteins. Our data support the hypothesis that the actin-binding determinants in native talin and other I/LWEQ module proteins are cryptic and indicate that the actin binding capacities of Talin1, Talin2, Hip1, and Hip12 are regulated by intrasteric occlusion of primary actin-binding determinants within the I/LWEQ module. We have also found that the I/LWEQ module contains a dimerization motif and stabilizes actin filaments against depolymerization. This activity may contribute to the function of talin in cell adhesion and the roles of Hip1, Hip12 (Hip1R), and Sla2p in endocytosis.

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Cell polarization is necessary for directed migration and leukocyte recruitment to inflamed tissues. Recent progress has been made in defining the molecular mechanisms that regulate chemoattractant-induced cell polarity during chemotaxis, including the contribution of phosphoinositide 3-kinase (PI3K)-dependent phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P 3 ] synthesis at the leading edge. However, less is known about the molecular composition of the cell rear and how the uropod functions during cell motility. Here, we demonstrate that phosphatidylinositol phosphate kinase type I␥ (PIPKI␥661), which generates PtdIns(4,5)P 2 , is enriched in the uropod during chemotaxis of primary neutrophils and differentiated HL-60 cells (dHL-60). Using time-lapse microscopy, we show that enrichment of PIPKI␥661 at the cell rear occurs early upon chemoattractant stimulation and is persistent during chemotaxis. Accordingly, we were able to detect enrichment of PtdIns(4,5)P 2 at the uropod during chemotaxis. Overexpression of kinase-dead PIPKI␥661 compromised uropod formation and rear retraction similar to inhibition of ROCK signaling, suggesting that PtdIns(4,5)P 2 synthesis is important to elicit the backness response during chemotaxis. Together, our findings identify a previously unknown function for PIPKI␥661 as a novel component of the backness signal that regulates rear retraction during chemotaxis.

The cytoskeletal protein talin serves as an essential link between integrins and the actin cytoskeleton in several similar, but functionally distinct, adhesion complexes, including focal adhesions, costameres, and intercalated disks. Vertebrates contain two talin genes, TLN1 and TLN2, but the different roles of Talin1 and Talin2 in cell adhesion are unclear. In this report we have analyzed Talin1 and Talin2 in striated muscle. Using isoform-specific antibodies, we found that Talin2 is highly expressed in mature striated muscle. Using mouse C2C12 cells and primary human skeletal muscle myoblasts as models of muscle differentiation, we show that Talin1 is expressed in undifferentiated myoblasts and that Talin2 expression is upregulated during muscle differentiation at both the mRNA and protein levels. We have also identified regulatory sequences that may be responsible for the differential expression of Talin1 and Talin2. Using GFP-tagged Talin1 and Talin2 constructs, we found that GFP-Talin1 targets to focal adhesions while GFP-Talin2 targets to abnormally large adhesions in myoblasts. We also found that ectopic expression of Talin2 in myoblasts, which do not contain appreciable levels of Talin2, dysregulates the actin cytoskeleton. Finally we demonstrate that Talin2, but not Talin1, localizes to costameres and intercalated disks, which are stable adhesions required for the assembly of mature striated muscle. Our results suggest that Talin1 is the primary link between integrins and actin in dynamic focal adhesions in undifferentiated, motile cells, but that Talin2 may serve as the link between integrins and the sarcomeric cytoskeletonin stable adhesion complexes in mature striated muscle.

Chemoattractants induce neutrophil polarization through localized polymerization of F-actin at the leading edge. The suppression of rear and lateral protrusions is required for efficient chemotaxis and involves the temporal and spatial segregation of signaling molecules. We have previously shown that the intracellular calcium-dependent protease calpain is required for cell migration and is involved in regulating neutrophil chemotaxis. Here, we show that primary neutrophils and neutrophil-like HL-60 cells express both calpain 1 and calpain 2 and that chemoattractants induce the asymmetric recruitment of calpain 2, but not calpain 1, to the leading edge of polarized neutrophils and differentiated HL-60 cells. Using time-lapse microscopy, we show that enrichment of calpain 2 at the leading edge occurs during early pseudopod formation and that its localization is sensitive to changes in the chemotactic gradient. We demonstrate that calpain 2 is recruited to lipid rafts and that cholesterol depletion perturbs calpain 2 localization, suggesting that its enrichment at the front requires proper membrane organization. Finally, we show that catalytic activity of calpain is required to limit pseudopod formation in the direction of chemoattractant and for efficient chemotaxis. Together, our findings identify calpain 2 as a novel component of the frontness signal that promotes polarization during chemotaxis. INTRODUCTIONNeutrophils are key components of the innate immune system, representing the first responders to inflammatory stimuli such as bacterial infections and tissue injury. The rapid recruitment of neutrophils to inflammatory sites requires a highly specialized form of cell migration in which polarity and movement are directed by spatial cues from a chemotactic gradient. Although the molecular mechanisms that regulate neutrophil chemotaxis remain poorly understood, recent progress indicates that this process is achieved by two distinct, but functionally linked events, including directional sensing and cell polarization (Devreotes and Janetopoulos, 2003;Franca-Koh and Devreotes, 2004).In professional migratory cells such as neutrophils, G protein-coupled cell surface receptors recognize external chemotactic gradients (Van Haastert and Devreotes, 2004;Huttenlocher, 2005) and are responsible for initiating the translation of spatial information about the chemotactic gradient into an internal gradient of signaling molecules that relay either "front"-or "back"-specific responses. For example, previous work has shown that phosphatidylinositol-3,4,5-trisphosphate (PIP 3 ) is asymmetrically recruited to the membrane adjacent to the highest concentration of chemoattractant (Servant et al., 2000;Kimmel and Parent, 2003) where it stimulates positive feedback mechanisms that promote the frontness signal. Subsequent reinforcement of frontness and backness signals dictate cell polarity, where pseudopod formation is biased according to the direction of the chemotactic gradient, with the leading edge characterized by the asymmetric...