Smooth Muscle Alpha-actinin Research Articles

The relationship between stress fiber-like structures and nascent myofibrils in cultured cardiac myocytes.

The topographical relationship between stress fiber-like structures (SFLS) and nascent myofibrils was examined in cultured chick cardiac myocytes by immunofluorescence microscopy. Antibodies against muscle-specific light meromyosin (anti-LMM) and desmin were used to distinguish cardiac myocytes from fibroblastic cells. By various combinations of staining with rhodamine-labeled phalloidin, anti-LMM, and antibodies against chick brain myosin and smooth muscle alpha-actinin, we observed the following relationships between transitory SFLS and nascent and mature myofibrils: (a) more SFLS were present in immature than mature myocytes; (b) in immature myocytes a single fluorescent fiber would stain as a SFLS distally and as a striated myofibril proximally, towards the center of the cell; (c) in regions of a myocyte not yet penetrated by the elongating myofibrils, SFLS were abundant; and (d) in regions of a myocyte with numerous mature myofibrils, SFLS had totally disappeared. Spontaneously contracting striated myofibrils with definitive Z-band regions were present long before anti-desmin localized in the I-Z-band region and long before morphologically recognizable structures periodically link Z-bands to the sarcolemma. These results suggest a transient one-on-one relationship between individual SFLS and newly emerging individual nascent myofibrils. Based on these and other relevant data, a complex, multistage molecular model is presented for myofibrillar assembly and maturation. Lastly, it is of considerable theoretical interest to note that mature cardiac myocytes, like mature skeletal myotubes, lack readily detectable stress fibers.

α-Actinin and Vinculin in Normal and Thrombasthenic Platelets

Recently, the contractile protein alpha-actinin was identified in normal human platelets by its antigenic cross-reaction with a monospecific antibody to purified muscle alpha-actinin. In this study, we extend that preliminary identification of platelet alpha-actinin. Amino acid analysis, one-dimensional peptide maps, and silver stain analysis on polyacrylamide gels demonstrate that human platelet alpha-actinin shows a greater degree of similarity to smooth muscle alpha-actinin than to striated muscle alpha-actinin. There is no evidence to suggest that alpha-actinin is a glycoprotein. In addition, we find that thrombasthenic platelets, which are deficient in glycoproteins IIb and IIIa (GPIIb and GPIIIa) contain normal amounts of alpha-actinin, confirming the recent finding that alpha-actinin and GPIIIa are different proteins in human platelets. We demonstrate that both normal and thrombasthenic platelets also contain vinculin, a 130,000-dalton polypeptide found in many cell types at sites of end-on attachment of microfilaments to the plasma membrane. Thus, the thrombasthenic defect in GPIIb and GPIIIa does not diminish the content of either alpha-actinin or vinculin.