Molecular Size and Shape of β-Connectin, an Elastic Protein of Striated Muscle1

Abstract

Connectin is an elastic protein of vertebrate striated muscle, and consists of doublet components, α and β (also called titins 1 and 2). In the present study, β-connectin isolated in the native state was investigated in order to characterize its molecular size and shape. The molecular weight was approximately 2.1 × 10⁶ (SDS gel elec-trophoresis) or 2.7 × 10⁶ (sedimentation equilibrium). The sedimentation coefficient (s_20,w⁰) 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 β-connectin is derived from β-connectin, which has an apparent molecular weight of 2.8 × 10⁶.