Connectin is an elastic protein of vertebrate striated muscle, and consists of doublet components, alpha and beta (also called titins 1 and 2). In the present study, beta-connectin isolated in the native state was investigated in order to characterize its molecular size and shape. The molecular weight was approximately 2.1 X 10(6) (SDS gel electrophoresis) or 2.7 X 10(6) (sedimentation equilibrium). The sedimentation coefficient (SO20, w) was 17S in 0.1 M phosphate buffer, pH 7.0. The intrinsic viscosity measured in an Ostwald-type viscometer was 1.8 dl/g. However, the viscosity was greatly dependent on the velocity gradient, and at a very low velocity gradient of 0.0007 s-1, a solution of connectin (0.3 mg/ml) showed a viscosity value of 17,000 cp. Flow birefringence measurements suggested a length distribution ranging from 300 to 450 nm. Electron microscopic observations revealed that connectin is a long flexible filament and the peaks of frequency of length distribution were at 150, 300, 450, and 600 nm. It was tentatively assumed that the connectin molecule is 300-400 nm long and 34-38 nm wide. It is likely that beta-connectin is derived from alpha-connectin, which has an apparent molecular weight of 2.8 X 10(6).

By sodium dodecyl sulfate (SDS)-gel electrophoresis, several types of connectin, an elastic protein of vertebrate skeletal muscle, different in their electrophoretic mobility, were detected in embryonic, neonatal and adult skeletal muscles of the chicken: embryonic a, ,8, ,6', neonatal a, and adult a, ,6, ,6'. The relative mobility was slower in the following order: embryonic a

Connectin filaments linking myosin filaments to Z lines in chicken skeletal muscle interact with myosin rods but not with S1 heads in vitro. An increase in turbidity of myosin rods, and heavy and light meromyosins was observed in the presence of con-nectin in KC] concentrations lower than 0.15 M, similarly to the case of intact myosin. The aggregate formation of myosin rods in the presence of connectin was observed by electron microscopy. It appears that the S2 portion ofmyosin does not bind to connectin.