Hypertrophy is a fundamental adaptive process employed by postmitotic cardiac and skeletal muscle in response to mechanical load. How muscle cells convert mechanical stimuli into growth signals has been a long-standing question. Using an in vitro model of load (stretch)-induced cardiac hypertrophy, we demonstrate that mechanical stretch causes release of angiotensin II (Ang II) from cardiac myocytes and that Ang II acts as an initial mediator of the stretch-induced hypertrophic response. The results not only provide direct evidence for the autocrine mechanism in load-induced growth of cardiac muscle cells, but also define the pathophysiological role of the local (cardiac) renin-angiotensin system.

Myosin 10 contains a region of predicted coiled coil 120 residues long. However, the highly charged nature and pattern of charges in the proximal 36 residues appear incompatible with coiled-coil formation. Circular dichroism, NMR, and analytical ultracentrifugation show that a synthesized peptide containing this region forms a stable single ␣-helix (SAH) domain in solution and does not dimerize to form a coiled coil even at millimolar concentrations. Additionally, electron microscopy of a recombinant myosin 10 containing the motor, the three calmodulin binding domains, and the fulllength predicted coiled coil showed that it was mostly monomeric at physiological protein concentration. In dimers the molecules were joined only at their extreme distal ends, and no coiled-coil tail was visible. Furthermore, the neck lengths of both monomers and dimers were much longer than expected from the number of calmodulin binding domains. In contrast, micrographs of myosin 5 heavy meromyosin obtained under the same conditions clearly showed a coiled-coil tail, and the necks were the predicted length. Thus the predicted coiled coil of myosin 10 forms a novel elongated structure in which the proximal region is a SAH domain and the distal region is a SAH domain (or has an unknown extended structure) that dimerizes only at its end. Sequence comparisons show that similar structures may exist in the predicted coiled-coil domains of myosins 6 and 7a and MyoM and could function to increase the size of the working stroke.Myosins make up a diverse superfamily of motor proteins (1). The human genome alone contains about 40 myosin genes (2). Of these, about one-third are "conventional" myosins (i.e. the well studied myosin 2), and the rest fall into about 10 different classes. The structure, properties and functions of the majority of myosin classes are poorly characterized and have largely been inferred from sequence comparisons rather than direct experiments on purified proteins (1-3).Muscle myosin 2 dimerizes through its ␣-helical coiled-coil tail. Therefore, it is commonly assumed that any myosin will also be dimeric if it contains a region predicted to be coiled coil. This assumption is dependent on the accuracy of coiled-coil prediction programs, such as COILS (4), PAIRCOIL, or MULTICOIL (5), which are also used by protein-fold prediction sites on the Web such as SMART (6).Although myosin 10 contains a region of predicted coiled coil (Fig. 1A), and is predicted to dimerize, this has not been determined experimentally. We noticed that part of the predicted coiled-coil domain of myosin 10 is highly enriched in charged residues (Fig. 1B). The proximal region consisting of 36 residues is particularly enriched in both positively and negatively charged residues, including the a and d positions of the heptad repeat (a-g) that are canonically hydrophobic residues in coiled coils (Fig. 1B). We suspected that this highly charged sequence is unlikely to form a coiled coil (7), suggesting that this part of myosin 10 may not be able to dimerize....

Quality control mechanisms prevent the cell surface expression of incompletely assembled multisubunit receptors such as the T cell receptor (TCR). The molecular chaperone function of calnexin (IP90, p88), a 90-kilodalton protein that resides in the endoplasmic reticulum (ER), in the retention of representative chains of the TCR-CD3 complex in the ER was tested. Truncation mutants of calnexin, when transiently expressed in COS cells, were exported from the ER and either accumulated in the Golgi or progressed to the cell surface. CD3 epsilon chains cotransfected with the forms of calnexin that were not retained in the ER exited the ER and colocalized with calnexin. Since engineered calnexin determined the intracellular localization of the proteins associated with it, it is concluded that calnexin interacts with incompletely assembled TCR components and retains them in the ER.