Focal adhesions (FAs) undergo myosinII-mediated maturation wherein they grow and change composition to modulate integrin signaling for cell migration, growth and differentiation. To determine how FA composition is modulated by myosinII activity, we performed proteomic analysis of isolated FAs and compared protein abundance in FAs from cells with and without myosinII inhibition. We identified FA 905 proteins, 459 of which changed in FA abundance with myosinII inhibition, defining the myosinII-responsive FA proteome. FA abundance of 73% of proteins was enhanced by contractility, including those involved in Rho-mediated FA maturation and endocytosis- and calpain-dependent FA disassembly. 27% of proteins, including those involved in Rac-mediated lamellipodial protrusion, were enriched in FA by myosinII inhibition, establishing for the first time negative regulation of FA protein recruitment by contractility. We focused on the Rac guanine nucleotide exchange factor, β-PIX, documenting its role in negative regulation of FA maturation and promotion of lamellipodial protrusion, FA turnover to drive cell migration.

Death-associated protein kinase (DAPK) is a death domain-containing serine/threonine kinase, and participates in various apoptotic paradigms. Here, we identify the extracellular signal-regulated kinase (ERK) as a DAPKinteracting protein. DAPK interacts with ERK through a docking sequence within its death domain and is a substrate of ERK. Phosphorylation of DAPK at Ser 735 by ERK increases the catalytic activity of DAPK both in vitro and in vivo. Conversely, DAPK promotes the cytoplasmic retention of ERK, thereby inhibiting ERK signaling in the nucleus. This reciprocal regulation between DAPK and ERK constitutes a positive feedback loop that ultimately promotes the apoptotic activity of DAPK. In a physiological apoptosis system where ERK-DAPK interplay is reinforced, downregulation of either ERK or DAPK suppresses such apoptosis. These results indicate that bidirectional signalings between DAPK and ERK may contribute to the apoptosis-promoting function of the death domain of DAPK.

Death-associated protein kinase (DAP-kinase) is a calcium/calmodulin-dependent serine/threonine kinase, and participates in various apoptosis systems. However, its apoptosis-promoting mechanism is poorly understood. Here, we demonstrate that DAP-kinase suppresses integrin-mediated cell adhesion and signal transduction, whereas dominant-negative interference of this kinase promotes adhesion. This effect of DAP-kinase is neither a consequence of apoptosis nor a result of decreased expression of integrins. Rather, DAP-kinase downregulates integrin activity through an inside-out mechanism. We present evidence indicating that this adhesion-inhibitory effect accounts for a major mechanism of the apoptosis induced by DAP-kinase. First, in growth-arrested fibroblasts, DAP-kinase triggers apoptosis in cells plated on fibronectin, but does not affect the death of cells on poly-l-lysine. Second, in epithelial cells, DAP-kinase induces apoptosis in the anoikis-sensitive MCF10A cells, but not in the anoikis-resistant BT474 cells. Most importantly, the apoptosis-promoting effect of DAP-kinase is completely abolished by enforced activation of integrin-mediated signaling pathways from either integrin itself or its downstream effector, FAK. Finally, we show that integrin or FAK activation blocks the ability of DAP-kinase to upregulate p53. Our results indicate that DAP-kinase exerts apoptotic effects by suppressing integrin functions and integrin-mediated survival signals, thereby activating a p53-dependent apoptotic pathway.

Death-associated protein kinase (DAPK) is a calmodulin-regulated serine/threonine kinase and possesses apoptotic and tumor-suppressive functions. However, it is unclear whether DAPK elicits apoptosis-independent activity to suppress tumor progression. We show that DAPK inhibits random migration by reducing directional persistence and directed migration by blocking cell polarization. These effects are mainly mediated by an inhibitory role of DAPK in talin head domain association with integrin, thereby suppressing the integrin–Cdc42 polarity pathway. We present evidence indicating that the antimigratory effect of DAPK represents a mechanism through which DAPK suppresses tumors. First, DAPK can block migration and invasion in certain tumor cells that are resistant to DAPK-induced apoptosis. Second, using an adenocarcinoma cell line and its highly invasive derivative, we demonstrate DAPK level as a determining factor in tumor invasiveness. Collectively, our study identifies a novel function of DAPK in regulating cell polarity during migration, which may act together with its apoptotic function to suppress tumor progression.

Ubiquitin chains are formed as structurally distinct polymers via different linkages, and several chain types including K33-linkage remain uncharacterized. Here, we describe a role for K33-polyubiquitination in protein trafficking. We show that the Cullin 3 (Cul3) substrate adaptor KLHL20 is localized to the trans-Golgi network (TGN) and is important for post-Golgi trafficking by promoting the biogenesis of TGN-derived transport carriers. The Cul3-KLHL20 ubiquitin E3 ligase catalyzes a nondegradable, K33-linked polyubiquitination on coronin 7 (Crn7), which facilitates Crn7 targeting to TGN through a ubiquitin-dependent interaction with Eps15. Blockage of K33-chain formation, Crn7 ubiquitination, or disruption of Crn7-Eps15 interaction impairs TGN-pool F-actin assembly, a process essential for generating transport carriers. Enforced targeting of Crn7 to TGN bypasses the requirement of K33-ubiquitination for TGN-pool F-actin assembly and post-Golgi trafficking. Our study reveals a role of KLHL20-mediated K33-ubiquitination of Crn7 in post-Golgi transport and identifies a cellular recognition mechanism for this ubiquitin chain type.

Focal adhesions (FAs) are complex plasma membrane-associated macromolecular assemblies that serve to physically connect the actin cytoskeleton to integrins that engage with the surrounding extracellular matrix (ECM). FAs undergo maturation wherein they grow and change composition differentially to provide traction and to transduce the signals that drive cell migration, which is crucial to various biological processes, including development, wound healing and cancer metastasis. FA-related signalling networks dynamically modulate the strength of the linkage between integrin and actin and control the organization of the actin cytoskeleton. In this review, we have summarized a number of recent investigations exploring how FA composition is affected by the mechanical forces that transduce signalling networks to modulate cellular function and drive cell migration. Understanding the fundamental mechanisms of how force governs adhesion signalling provides insights that will allow the manipulation of cell migration and help to control migration-related human diseases.

Death-associated protein kinase (DAP kinase) is a proapoptotic, calcium/calmodulin-dependent serine/threonine kinase. Here, we report that DAP kinase phosphorylates the regulatory light chain of myosin II (MLC) both in vitro and in vivo, and that this phosphorylation occurs preferentially at residue Ser19. In quiescent fibroblasts, DAP kinase stabilizes stress fibers through phosphorylation of MLC, but it is dispensable for the formation of peripheral microfilament bundles. This cytoskeletal effect of DAP kinase occurs before the onset of apoptosis and does not require an intact death domain. In addition, DAP kinase is required for serum-induced stress-fiber formation, which is associated with the upregulation of its catalytic activity. Despite being both sufficient and necessary for the assembly or maintenance of stress fibers, DAP kinase is incapable of stimulating the formation of focal adhesions in quiescent cells. Moreover, it promotes the disassembly of focal adhesions but not stress fibers in cells receiving serum factors. Together, our results identify a novel and unique function of DAP kinase in the uncoupling of stress fibers and focal adhesions. Such uncoupling would lead to a perturbation of the balance between contractile and adhesion forces and subsequent cell detachment, which might contribute to its pro-apoptotic activity.

Directed cell migration is an important step in effective wound healing and requires the dynamic control of the formation of cell-extracellular matrix interactions. Plasma fibronectin is an extracellular matrix glycoprotein present in blood plasma that plays crucial roles in modulating cellular adhesion and migration and thereby helping to mediate all steps of wound healing. In order to seek safe sources of plasma fibronectin for its practical use in wound dressing, we isolated fibronectin from human (homo) and porcine plasma and demonstrated that both have a similar ability as a suitable substrate for the stimulation of cell adhesion and for directing cell migration. In addition, we also defined the N-glycosylation sites and N-glycans present on homo and porcine plasma fibronectin. These N-glycosylation modifications of the plasma fibronectin synergistically support the integrin-mediated signals to bring about mediating cellular adhesion and directed cell migration. This study not only determines the important function of N-glycans in both homo and porcine plasma fibronectin-mediated cell adhesion and directed cell migration, but also reveals the potential applications of porcine plasma fibronectin if it was applied as a material for clinical wound healing and tissue repair.