Shapes, domain organizations and flexibility of laminin and fibronectin, two multifunctional proteins of the extracellular matrix

Abstract

Laminin from a mouse tumor basement membrane and fibronectin from human blood plasma were examined by electron microscopy using rotary shadowing and negative staining and by transmission scanning electron microscopy of unstained samples. Laminin was visualized as a rigid, asymmetric cross consisting of a long (77 nm) and three apparently identical short (36 nm) arms. The rod-like arms (diameter about 2 nm) terminated in globular units (diameter 5 to 7 nm). Additional globules were found near the terminal units in the short arms. A large pepsin-resistant fragment of laminin appeared as a rigid structure with three arms (length 26 nm, preferred angle 90 °), which presumably represented parts of the three short arms of laminin. Fibronectin could be visualized as two identical strands (length 61 nm, diameter about 2 nm), which did not reveal distinct globular units. These strands very likely comprised single peptide chains connected to each other at one end, enclosing a fixed angle of about 70 °. Electron microscopy also indicated a limited flexibility of the arms of both laminin and fibronectin, comparable to the stiffness of tropomyosin or DNA. The electron microscopic images of the shapes and dimensions of laminin, of fragments of laminin, and of fibronectin are consistent with the specific molecular weights and with the hydrodynamic properties determined in solution. The arms of fibronectin showed three distinct regions at which preferential bending occurred. These sites apparently correspond to flexible segments connecting more compact domains previously identified in biochemical studies. No sites of preferential bending were visible in the arms of laminin. Although laminin and fibronectin have some similar biological activities (binding of cells, collagen, glycosaminoglycans), the corresponding functional domains are differently arranged in the two molecules.