Background: Cardiac myosin regulatory light chain (RLC) phosphorylation alters cardiac muscle function.Results: Phosphorylation affects mechanical parameters of cardiac muscle contraction during shortening.Conclusion: Phosphorylation impacts mechanical function of cardiac muscle and is altered during cardiac disease.Significance: Understanding RLC regulation by phosphorylation in cardiac muscle contraction is crucial for understanding changes in disease.

The amyloid precursor protein (APP) and the presenilins 1 and 2 are genetically linked to the development of familial Alzheimer disease. APP is a single-pass transmembrane protein and precursor of fibrillar and toxic amyloid-␤ peptides, which are considered responsible for Alzheimer disease neurodegeneration. Presenilins are multipass membrane proteins, involved in the enzymatic cleavage of APP and other signaling receptors and transducers. The role of APP and presenilins in Alzheimer disease development seems to be related to the formation of amyloid-␤ peptides; however, their physiological function, reciprocal interaction, and molecular mechanisms leading to neurodegeneration are unclear. APP and presenilins are also involved in multiple interactions with intracellular proteins, the significance of which is under investigation. Among the different APP-interacting proteins, we focused our interest on the GRB2 adaptor protein, which connects cell surface receptors to intracellular signaling pathways. In this study we provide evidence by co-immunoprecipitation experiments, confocal and electron microscopy, and by fluorescence resonance energy transfer experiments that both APP and presenilin1 interact with GRB2 in vesicular structures at the centrosome of the cell. The final target for these interactions is ERK1,2, which is activated in mitotic centrosomes in a PS1-and APP-dependent manner. These data suggest that both APP and presenilin1 can be part of a common signaling pathway that regulates ERK1,2 and the cell cycle.

Cell membrane deformations are crucial for proper cell function. Specialized protein assemblies initiate inward or outward membrane deformations that the cell uses respectively to uptake external substances or probe the environment. The assembly and dynamics of the actin cytoskeleton are involved in this process, although their detailed role remains controversial. We show here that a dynamic, branched actin network is sufficient to initiate both inward and outward membrane deformation. The polymerization of a dense actin network at the membrane of liposomes produces inward membrane bending at low tension, while outward deformations are robustly generated regardless of tension. Our results shed light on the mechanism cells use to internalize material, both in mammalian cells, where actin polymerization forces are required when membrane tension is increased, and in yeast, where those forces are necessary to overcome the opposing turgor pressure. By combining experimental observations with physical modeling, we propose a mechanism that explains how membrane tension and the architecture of the actin network regulate cell-like membrane deformations.How the same branched actin structure can be responsible for the initiation of filopodia, which are outward-pointing membrane deformations, as well as endocytic invaginations that deform the membrane inward, is what we want to address in this paper. Such a question is difficult to investigate in cells that contain redundant mechanisms for cell deformation. Actin dynamics triggered at a liposome membrane provide a control on experimental parameters such as membrane composition, curvature and tension, and allow the specific role of actin dynamics to be addressed. We unambiguously show that the same branched actin network is able to produce both endocytosis-like and dendritic filopodia-like deformations. With a theoretical model, we predict under which conditions the stress exerted on the membrane will lead to inward and/or outward pointing membrane deformations. Combining experiments and theory allows us to decipher how the interplay between membrane tension, actin dynamics, and actin network structure produces inward or outward membrane deformations. Membrane deformations: tubes and spikesLiposomes are covered with an activator of the Arp2/3 complex, pVCA, the proline rich domain-verprolin homology-central-acidic sequence from human WASP, which is purified with a streptavidin tag, and that we call hereafter S-pVCA. A branched actin network grows at their surface when placed in a mixture containing monomeric actin, profilin, the Arp2/3 complex and capping protein (CP) ("reference condition", Methods and Fig. 1a). Strikingly, the membrane of liposomes is not smooth, but instead displays a rugged profile: membrane tubes, hereafter called "tubes", radiate from the liposome surface and extend into the actin network (Fig. 1b), even when comet formation has occurred 7, 8 (Supplementary Fig. 1a). The initiation of these tubes is reminiscent of early stage of endocytosis. Interesting...